AMD Opteron UP 170 Dual Core San Diego 2.0GHz 2MB Cache (Socket 939) - Retail (CP-148-AM) -
http://www.overclockers.co.uk/acatalog/AMD_Athlon_MP.html
If your friend has some knowlege on overclocking he will like s939 Opteron chips (& some of them are actually a little cheaper then the athlon chips & since they are intaillay work station/server chips they actually have there silcon thurthly checked by AMD, they also do fairly nice overclocks).
Personally I'd try getting a dual core chip for future proofing as with all the dual core patches for games & apps coming out these days (points at quake4).
AMD Athlon 64 X2 Dual Core 4200+ (Socket 939) - Retail (ADA4200BVBOX) (CP-126-AM) -
http://www.overclockers.co.uk/acatalog/AMD_64_X2.html
DFI LanParty NF4 SLI-DR EXPERT (Socket 939) PCI-Express Motherboard (MB-018-DF) -
http://www.overclockers.co.uk/acatalog/Socket_939.html
Mushkin 2GB DDR HP3200 High Performance Dual Channel Kit (2x1GB) CAS2 (991434) (MY-005-MK) -
http://www.overclockers.co.uk/acatalog/Mushkin.html
Leadtek GeForce 7800GT 256MB GDDR3 VIVO TV-Out/Dual DVI (PCI-Express) - Retail (GX-069-LT) -
http://www.overclockers.co.uk/acatalog/7800__Series.html
Western Digital Caviar SE16 320GB 3200KS SATA-II 16MB Cache - OEM (HD-070-WD) -
http://www.overclockers.co.uk/acatalog/westerndigital_sata.html
Antec Sonata II Piano Black Quiet Case - 450W Smart Power PSU (CA-040-AN) -
http://www.overclockers.co.uk/acatalog/Antec_Cases.html
Total cost comes in at about 824.7 (I don't have your currency sign on my keyboard). Imo I wouldn't bother with a 7800gtx & go with a gt instead because no one will really notice the difference between the two. In a few days nvidia should release the 7900 series (GT 24pipes 450core/1300memory, GTX 24pipes, 650core/1600memory). Ofcourse they were aiming for higher clocks but that's from nvidia's reference design for the cards & some partners will have factory overclocked versions that should ship at clocks of 690 - 710 on the core. These cards are a die shrink (90nm comapred to 7800series 110nm) & since it is smaller, it means it will produce less heat and consume less power (in theory atleast). My prediction is 7800cards will be discounted as most retailers will want to get rid of them when these cards are launched (nvidia is planning to launch them at cebit on march 9th, same day as my birthday :'). They should also have good supply of these cards at launch according to nvidia (just like 7800series did at launch, except for the GTX 512 with it's limited avalibility).
Personally though I would just go buy a 7800gt and wait for the next generation of cards considering what vista is going to do to us :'(.