Loose rivets,
It all comes down to how much you want to spend.
The AthlonXP 2500+ is a great CPU that overclocks like crazy (I keep a 500Mhz overclock on mine with some serious cooling). Pair it with an nForce2 Ultra 400 Mobo like the:
ASUS A7N8X-E Deluxe
and the MSI K7N2 Delta2 Platinum
If you are looking at getting an Athlon64, then look at the Socket 939's since they are changing from the Socket 754 Athlon64's to the Socket 939 Athlon64's. A couple of good nForce3 Ultra mobos to look at are the:
MSI K8N Neo2 Platinum
and the Gigabyte GA-K8NSNXP-939
When it comes to Vid Cards, two "Best Bang for the Buck" options are the:
ATI Radeon 9800Pro 128Mb
and the MSI GeForce FX 5900XT 128Mb
Take Care,
Richard