Erm, I doubt you'll notice a ground breaking difference. And the XP 3000+ isn't a very good processor. You should think about two things:
1. Get a XP 2500+ and overclock it to a 3200+
2. Buy the 3200+ because it runs with a 200Mhz FSB, where the 3000+ still runs on 166Mhz.
The former is obviously much cheaper, but you may need a better heatsink/fan, and overclocking of course.
Edit: come to think of it, if you can't run the game on high settings with a 6600GT and 1 gig of ram, then perhaps it is indeed your processor that's slowing you down. What kind of ram you got?
ASUS A7N8X-E Deluxe
AMD Athlon XP 3200+
OCZ 1GB Platinum PC3200 Rev2 Dual Channel
ATI Radeon 9800 Pro
Enermax Noisetaker 485W
2 x 80GB SATA Maxtor Diamondmax 10 on RAID 0