The highest the socket A got to was the 3200+. And, while the socket was the kick-butt CPU of the day, that day has long passed. You may be able to find a 3000+ or a 3200+ if you're lucky (search for it on here, as a few had them for sale a few months back). You're best bet is to move on up.
While Intel's C2D is top dog right now, and AMD's AM2 is the current stock, you'd do well to look into AMD's 939 line. The prices have really fallen on them since they have reached the end of their life as far as AMD is concerned... and you obviously don't have a problem using non-cutting edge hardware
. Plus, it uses DDR RAM, which is what you already have.
A 3000+ s939 can be had online for a shade over $50. A 3800+ is just over $100. You can go dual core with an Opteron 165 for $155+.
As for motherboards, you have the whole spectrum open before you. Since 939 is older tech, the good boards are well known and the lemons are as well. Look for a board with an nForce4 chipset. I'd recommend an Ultra chipset, unless you absolutely have to have SLI. The DFI LanParty UT NF4 Ultra-D can be had for $109.99 from Directron, while the ABIT KN8 Ultra can be found for less than $90.
The choice is your's, of course. But the move to a 64-bit CPU brings speed that will blow your mind if you've been on a 32-bit CPU.