Hmm, well the AMD 64 3000+ Venice core would be a great addition to that motherboard. It wouldn't set you back alot, but its a solid performer and can be overclocked alot if you wanted it to be.
I have a AMD 64 3500+ but i WISH that I went for the AMD 64 3000+ as I wouldn't have seen much difference, and then overclocked it once I had the chance. If I did that, I would have been able to afford a better video card over my previous 9800Pro and would have got a PCI-E motherboard too...
Its soley up to you though. I mean if you had more money, you could go for the new X2's with dual cores for extra speed, but you'd need alot of money to do so... actually...I think you might need two CPU spaces on the motherboard to do that anyway...I'm not sure...
Anyhow, if you had money then, you could go for the higher end CPU's like the FX's yet, you don't really need all that performance, and my computers great for doing what I do, and I do alot of editing, and 3D work and also multitask rather alot to.
So basically, its up to you
Just give yourself a price range and aim for it. Anything from the AMD 64 3000+ up will be great for the system.
I do advise you go for a 64 bit Venice core though since you'll get alot of performance from them, and your PC will be ready for when 64 bit applications come off the market.