The 9800GTX is actually (spec-wise, I haven't seen benchmarks)[slih better than the HD4850:
9800GTX<table border="0" cellpadding="1" cellspacing="0" width="100%"><tbody><tr><td class="card-info-column1">
Manufacturer:</td><td class="card-info-column2">nVidia</td></tr> <tr><td class="card-info-column1">
Series:</td><td class="card-info-column2">GeForce 9</td></tr> <tr><td class="card-info-column1">
GPU:</td><td class="card-info-column2">
G92 </td></tr> <tr><td class="card-info-column1">
Release Date:</td><td class="card-info-column2">2008-03-31</td></tr> <tr><td class="card-info-column1">
Interface:</td><td class="card-info-column2">PCI-E 2.0 x16</td></tr> <tr><td class="card-info-column1">
Core Clock:</td><td class="card-info-column2">675 MHz</td></tr> <tr><td class="card-info-column1">Shader Clock:</td><td class="card-info-column2">1688 MHz</td></tr> <tr><td class="card-info-column1">
Memory Clock:</td><td class="card-info-column2">1100 MHz (2200 DDR)</td></tr> <tr><td class="card-info-column1">
Memory Bandwidth:</td><td class="card-info-column2">
70.4 GB/sec</td></tr> <tr><td class="card-info-column1">
FLOPS:</td><td class="card-info-column2">432.128 GFLOPS</td></tr> <tr><td class="card-info-column1">
Pixel Fill Rate:</td><td class="card-info-column2">
10800 MPixels/sec</td></tr> <tr><td class="card-info-column1">
Texture Fill Rate:</td><td class="card-info-column2">
43200 MTexels/sec</td></tr></tbody></table>
HD4850
<table border="0" cellpadding="1" cellspacing="0" width="100%"><tbody><tr><td class="card-info-column1">
Manufacturer:</td><td class="card-info-column2">ATi</td></tr> <tr><td class="card-info-column1">
Series:</td><td class="card-info-column2">Radeon HD 4k</td></tr> <tr><td class="card-info-column1">
GPU:</td><td class="card-info-column2">
RV770 </td></tr> <tr><td class="card-info-column1">
Release Date:</td><td class="card-info-column2">2008-06-25</td></tr> <tr><td class="card-info-column1">
Interface:</td><td class="card-info-column2">PCI-E 2.0 x16</td></tr> <tr><td class="card-info-column1">
Core Clock:</td><td class="card-info-column2">625 MHz</td></tr> <tr><td class="card-info-column1">Shader Clock:</td><td class="card-info-column2">625 MHz</td></tr> <tr><td class="card-info-column1">
Memory Clock:</td><td class="card-info-column2">993 MHz (1986 DDR)</td></tr> <tr><td class="card-info-column1">
Memory Bandwidth:</td><td class="card-info-column2">63.552 GB/sec</td></tr> <tr><td class="card-info-column1">
FLOPS:</td><td class="card-info-column2">
1000 GFLOPS</td></tr> <tr><td class="card-info-column1">
Pixel Fill Rate:</td><td class="card-info-column2">10000 MPixels/sec</td></tr> <tr><td class="card-info-column1">
Texture Fill Rate:</td><td class="card-info-column2">25000 MTexels/sec</td></tr></tbody></table>
Bolded means better than the other card. Although I am uncertain about the 1000GFLOPS of the Ati.
However, check the GTX 280. It beats both cards by a mile. It is twice as strong as the 9800GTX is. The difference between the 8XXX series and 9XXX series is small, but between those and the new 280...
GTX 280
<table border="0" cellpadding="1" cellspacing="0" width="100%"><tbody><tr><td class="card-info-column1">
Manufacturer:</td><td class="card-info-column2">nVidia</td></tr> <tr><td class="card-info-column1">
Series:</td><td class="card-info-column2">GeForce GTX 200</td></tr> <tr><td class="card-info-column1">
GPU:</td><td class="card-info-column2">
GT200 </td></tr> <tr><td class="card-info-column1">
Release Date:</td><td class="card-info-column2">2008-06-16</td></tr> <tr><td class="card-info-column1">
Interface:</td><td class="card-info-column2">PCI-E 2.0 x16</td></tr> <tr><td class="card-info-column1">
Core Clock:</td><td class="card-info-column2">602 MHz</td></tr> <tr><td class="card-info-column1">Shader Clock:</td><td class="card-info-column2">1296 MHz</td></tr> <tr><td class="card-info-column1">
Memory Clock:</td><td class="card-info-column2">1107 MHz (2214 DDR)</td></tr> <tr><td class="card-info-column1">
Memory Bandwidth:</td><td class="card-info-column2">
141.696 GB/sec</td></tr> <tr><td class="card-info-column1">
FLOPS:</td><td class="card-info-column2">933.12 GFLOPS</td></tr> <tr><td class="card-info-column1">
Pixel Fill Rate:</td><td class="card-info-column2">
19264 MPixels/sec</td></tr> <tr><td class="card-info-column1">
Texture Fill Rate:</td><td class="card-info-column2">
48160 MTexels/sec</td></tr></tbody></table>
GTX 260
<table border="0" cellpadding="1" cellspacing="0" width="100%"><tbody><tr><td class="card-info-column1">
Manufacturer:</td><td class="card-info-column2">nVidia</td></tr> <tr><td class="card-info-column1">
Series:</td><td class="card-info-column2">GeForce GTX 200</td></tr> <tr><td class="card-info-column1">
GPU:</td><td class="card-info-column2">
GT200 </td></tr> <tr><td class="card-info-column1">
Release Date:</td><td class="card-info-column2">2008-06-16</td></tr> <tr><td class="card-info-column1">
Interface:</td><td class="card-info-column2">PCI-E 2.0 x16</td></tr> <tr><td class="card-info-column1">
Core Clock:</td><td class="card-info-column2">576 MHz</td></tr> <tr><td class="card-info-column1">Shader Clock:</td><td class="card-info-column2">1240 MHz</td></tr> <tr><td class="card-info-column1">
Memory Clock:</td><td class="card-info-column2">999 MHz (1998 DDR)</td></tr> <tr><td class="card-info-column1">
Memory Bandwidth:</td><td class="card-info-column2">
111.888 GB/sec</td></tr> <tr><td class="card-info-column1">
FLOPS:</td><td class="card-info-column2">714.24 GFLOPS</td></tr> <tr><td class="card-info-column1">
Pixel Fill Rate:</td><td class="card-info-column2">
16128 MPixels/sec</td></tr> <tr><td class="card-info-column1">
Texture Fill Rate:</td><td class="card-info-column2">
36864 MTexels/sec</td></tr></tbody></table>
Downside of the GTX? Not very future proof. They only have DirectX 10.0, while the ATI has DirectX 10.1. And it probably won't take long before DirectX 11.0 is released. Still, power and spec-wise, the GTX are very future proof, and can probably be used for the coming years unless you
really want that DirectX 11.0.
My advice: Got money? Get the GTX 280(450 dollars), got a bit less money? Get the 260(300 dollars). Don't want to pay that much? Get the HD4850 (160/190 dollars) or the 9800GTX for 170 dollars. Is even that too much? Get the slightly weaker 8800GT for 120-ish dollars.