Regarding your cooling problem, get a software sensor to test your temperatures, use something like Motherboard Monitor 5.
Now for your cpu, since you don't want to get a new board or a new comp, then you'll have to get a better XP processor. When you say you can't use XP 3200+, how do you know that? I would assume your socket supports up to 3200+, which is the fastest XP processor out.
Whatever you do, be sure to solve that cooling problem first.
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