No, they have a bigger L2 cache on SOME processors, which isn't that big of a performance gain. And the P4 64 isn't coming out for at LEAST a year, probably more, around the time dual-core hits.
Temperamental--seriously, consider building your own system. You know most of those systems? Won't play a damned thing. The video card on all of them slows down the performance. Not to mention they cost about 4/3's more than building one yourself would. That's 300 dollars you'd be saving right there, buddy. For that money you could get yourself an awesome videocard--9800 pro, for one.
If you're NOT going to get a new computer, I suggest
this one
Get a new video card (you could even afford a 6800GT), maybe new RAM, but the processor is okay. I'd suggest, for the lowest budget possible, 9800np or 9800 pro.
Why do I suggest the slower clock?
1) You're going for a prebuilt, but you should be able to overclock (a setting in the BIOS). Getting 200-400 MHz more outta this thing would be a breeze, if so.
2) I myself only run a 2400+, yet that hinders my gaming in very, very small ways. I run Doom 3 at 1024x768 8xaa 16xaf, on high, with 512MB of PC2100, and a 6800GT. No big performance hit--30-60 fps constant.