which 8600 is better

I have ut3 and i run it at an average of 20fps right now

motherboard asus m2a-vm

nvidea 8500gt 512mg

amd x2 5000+ 2.6mhz

3gigs of ram

xp service pack2

Q.1 I am planing to get one of these 2 gpus. One is a 512 the other is 256 what will be the biggerst difference and which one can handle more overclicking wise?

http://www.buy.com/prod/xfx-geforce-8600gt-512mb-128-bit-ddr3-540mhz-pci-e-dual-dvi-tv-out-sli/q/loc/101/206574343.html

http://www.buy.com/prod/xfx-geforce-8600gts-xxx-edition-256mb-128-bit-ddr3-730mhz-pci-e-dual/q/loc/101/204450362.html

Q.2 How much can I take ut3 up to now?

I think you should get the 8600gts. I have it but 512mb and it's fine for me. I don't think that the mb will matter that much since it's mostly about the clock speed. I never even overclocked mine and I could play COD4 at max with a single core cpu so 256mb should be good.