I have the A7V8x as well. I love it!
The A7N8x would probably be your best choice. I'm probably the biggest fan of buying a kickass system board, as it is the primary building block for your entire computer. Even at the sacrifice of processor speed, i.e. I had the choice of going with an A7V333 w/ a 2200+ or an A7V8x w/ a 2100+...this was when their price difference was alot more...And I am not regretful of my decision.
As you have 2 sticks of RAM, you can take advantage of the nForce 2's Dual Channel DDR system... you can learn more about that in a few of the threads here.
As for VIA, I'd reccomend waiting for the KT400a before you get a VIA chipset.
Since your budget it $500 (

) I would reccomend this:
Prices from www.pricewatch.com
Athlon XP 2700+ (333 FSB) - $328
A7N8x - $115
Total - $443 w/ shipping...
That's if you decide to keep your current RAM. Which would not do that combo justice...
Athlon XP 2400+ - $160
You know what sucks? There's a price difference of $116 from the 2400+ to the 2600+
A7N8x Deluxe (Decked Out!) - $140
2x 256MB Either Micron or Samsung DDR400 - $140
That leaves you $60...w/ shipping
I would highly reccomend this 2nd choice!
Good deal eh?
