I guess the bigger question is what else are you looking to do?
With a budget of $250 you're well above what you need to reach the 120fps mark in any Source Game. As good as all the mods for HL:2 are, quite frankly at this point the engine is becoming dated and any of the new cards will have no problem running TF2/CSS with ease.
A good rule of thumb is, 30 fps is the lowest playable framerate, anything above 60 fps and you won't be able to tell the difference anyway.
So a couple different options.
If you want to save money and get a setup with 100+ framerates, grab an
ATI Radeon 4870 1gig or an
nVidia GTX 260 (216). I think recently the 4870 has been retailing for less so that may be a better option right now, though both are excellent cards. This will run you about $140-160, will give you excellent framerates for HL:2 and gives you the option to buy an additional card down the road for less and setup SLI/Crossfire.
If you want a better Single Card solution, the ATI
Radeon 4890, 4850x2 that have been mentioned, as well as the
nVidia GTX 275 are all strong cards that will run you around $190-240. This is well more than you need for 120 fps in TF2/CSS but it will be better suited to run more intensive games like Crisis.
If you're just trying to flex against the other players on the server, you could look at setting up a crossfire/SLI solution with dual
ATI Radeon 4850's, 4870's or an SLI configuration with nVidia
GTX 250's or 260's. This would be stretching your budget a bit on the high end or right around $220+ on the low end, but would probably give you the most performance right now.
Also, if you're looking for a beastly gaming rig, I'd split for a new Core i7 board a proc. The prices have actually come down decently to where a mobo will run you about $200-300 and the
Core i7 920 will run you about $280. But the 920 will outpace pretty much everything other than higher end C2 Quads and leave you open for current gen upgrades.