8800GTS SC 320 or 8800GTS 640

The extra mb is only going to help in texture rich games or at insainly high res the two cards perform verry similarly in bench marks and reviews i have seen.
Whats the swap going to cost?, if its minimal then it could be a way of getting a card that may last longer as it will be able to play games that demand more mb in future but by then dx10 will prob have moved on and we will be posting about a whole new range of cards anyway.
I know its too late now but for future ref kep in mind that you have the option of buying the stock card and ocing yourself which would have been cheaper for you this time round,(obviously)

You can overclock it quite a bit.

I'm pretty sure it'll come clocked a little higher than 500 anyway; the last 3 Evga cards I've used were clocked higher than their specs indicated. The 7800 GT I bought was clocked at 470 as opposed to 400; I had to underclock it to 460 or so to get rid of artifacts in certain games.

You should be able to push it above 600/1800 (I set my fan to 100%). The 8800 I tested went even higher than that pretty easily.

I don't think the 640MB version will be worth the extra money anytime soon... unless you run everything at high resolution with AA and AF enabled. I have the Superclocked 320MB version, and it's an awesome card. It also overclocks well beyond what it does out of the box... I have gotten it running at 675/1150 (core/memory) with some supplemental cooling, although I prefer to leave it around 630/1000 most of the time. I highly recommend the card.