HD 4850 1 GB or 512 MB ?

You probably won't need the 1GB unless you're planning on getting a 24" or larger monitor. If you're going to be playing games in resolutions higher than 1680x1050, then I'd go with the 1GB version. That's my opinion anyway.

I agree with arson94...

Blatantly 1 gig

Yea I'd definitely go with the 1GB, but I got the extra $50 to burn in such a situation and I wouldn't think twice about it. Actually, I'd get the 4870 1GB if anything. But, it's also Crysis. It also probably won't make too much a difference unless he gets a higher end dual core or any quad core as Crysis is one of those CPU intensive bastards, isn't it?

If an extra $50 is going to break the bank, get the 512MB. If it's nothing, get the 1GB. I'd even go further and say that if you're playing higher end games like Crysis, spend $300 and get the 4870 1GB. I'd hump the 4870 1GB if I could. I would absolutely cherish the 4870 X2 'till death do us part but that whore is $550 or so and I just can't part with that kind of cash for a single graphics card.

I have a 4870x2 with a q6660 at 3.2ghz and its performance is beastly. get 55fps with everything maxed on crysis - but i am playing at 1680x1050. Annoyingly, my mate gets about 48fp with everything maxed on crysis and he uses a 4870 1 gig. the 4870x2 doesn't do that much better at this resolution - you need a bigger screen to make it count.
I agree with arson94 - I reckon that the 4870 1 gig would be the sweet spot in performance vs. price

get the 1gig, but you will only see a difference if you run a huge monitor with over 1900x1200. Smaller than that the 512m is overkill. I think it only uses 128 or 256mb of it anyways.

High-resolution isn't the only thing extra memory is good. It also helps performance when there is an increase in textures and and such. The 1gb model is a little more future-proofed for new games than the 512mb version.