IMO - get a new vid card. While the GF4 4200 is not bad - when are you going to be able to spend money on the computer again? Bite off the whole enchilada now.

I see by the last system specs you posted, you realized the incremental MHz gain costs way too much as you climb the scale.
example: 2.6C GHz 800FSB PIV is 198.00, while a 3.06 is 379.00 (at newegg)
Is 0.46 GHz really worth an additional 201 bucks?
Or, Athlon Barton 2600 is 98 bucks while the Barton 3200 is 445.00 (newegg again) - is 0.6 pseudo-GHz worth 347 bucks?
IMO, NO - better to spend the differential on a Radeon 9700Pro or 9800Pro. Your current Vid card will be choking your new system. Not to mention that its not DX9 compliant, and I *think* AH2 will be DX9 game?
512 MB memory is a decent compromise for now, and nothing's easier to add to a system (and reasonably cheaply!) than RAM.
BTW - what size hard drive?
BB