Save up a bit and get a good overhaul. Get a 8800gts 320mb since you're using a 19", they're all dropping prices now, you'll be able to play almost any current game out there at 1240x780 maxed out. There's are also dirt cheap 20"+ monitors with higher res, but if you get anything 20"+ you'll need a 640mb. But either card will last you alot longer, for only a few 100$ more.
That'll be around 400-600$ for a 8800gts 320 and a decent 20", depending on where you buy your stuff.
An 8600gt will be fine too if you don't wanna spend that cash, and if you're keen on playing the games you already have at high-max.
Definetly get 2x1gb ram.
DDR2 ram is also dirt cheap.
You'll also need a slightly higher psu, but the corsair 450w would be able to handle a single card setup, and its about 70$. Or you can get the Antec Sonata III case w/ 500w psu for about 30-40$ more, both are good.
I'd change the cpu and mobo too..

Check your current card or bring in your pc in to a tech until then, it might be shot.