Give us a price range and desired resolution, and we can recommend something. I can say that a X600 of any sort won't do. In US prices, the best deals in PCIe cards are a 6600GT 128MB for <$200 right now, and a X800XL 256MB for <=$300 when it arrives in January.
The X800XL looks particularly fantastic for the price, a full 16-pipe card with 256MB of fast RAM. It even manages to make the otherwise nice 8-pipe, 128MB 6600GT seems slightly overpriced in comparison.
The X700XT won't be sold (at all), and the 6600GT is as good a deal or better, anyway. The X700P isn't as fast as a 6600GT, and you seem to be after speed first.
You'll also want to upgrade to 1GB RAM, especially for online gaming. Heck, 512MB PC3200 sticks go for as little as $45AR, so you can probably upgrade to a 6600GT and two 512MB PC3200 sticks for not much more than you can get for selling your current rig (CPU, MB, RAM, & 4600).
If you read the benchmarks in my previous post, you probably wouldn't still be asking, "What card," but rather, "What's the best deal on card X?"
Either way, make sure you have a decent PSU to power that P4. A 6600GT, 6800, or X800XL probably won't use that much power by itself, but P4Es aren't exactly stingy with electricity.