Upgrading 8800GTS, Budget: around £100 ($152)

Hello everyone

Im looking to upgrade my system to get better FPS on global agenda (Mainly) and my other steam games (e.g left4dead, TF2).
unfortunately im very constrained with the amount of money i can spend atm.

im not too concerned about playing at ultra-high resolutions or extreme anti-aliasing, I would just like something better than the 30 average FPS im getting now (less in large maps with lots of action) with all settings on low.

my spec is:

Asus P5N32-E SLI PLUS
Intel E6750 Core 2 Duo @ 2.66mhz
Inno3D 8800GTS 320mb
4GB DDR2 800mhz ram (Crucial Ballistix)
Thermaltake Toughpower 850W
Phillips 230C 23" monitor
Windows 7 Ultimate 64bit

all at stock speeds, no overclocking

Im wondering about getting a GTX250 1GB .

Would this give me a decent enough performance increase for my money do you think ? or would any perfomance gains be largely negated by my current CPU.

are there other cards/upgrades that you think would make a better choice ? (bearing in mind my lack of money atm) I might be able to spend 10-20 more if there is a lot more mileage to be had out of it.

any advice you could give will be very much appreciated

thank you very much

obuk2k