Well, your processor ain't so good, and that's a big part of the problem...HL is not made for such large open maps with many people flying around, it takes a lot of processor speed to do all that. Frankly with an 800MHz you're not going to do much better. I would look at the Tweaks section of the ESF Chat area (it's a sticky) to try and speed up your HL. Remember to take off all the nice graphics options under customize in HL, like high quality models and effects, etc., as much as you can stand to do without.