Ignore the 1 gig of memory. As we have hashed out time after time on these forums, a gig of memory on a video card is useless UNLESS you plan on putting a couple of them in SLI, then the 2 cards together can muster enough rendering power to start making use of over 512 meg of memory.
A single card in this class will start to fall on it's face by the time you turn the detail level, or the resolution up enough to begin using more than 512meg of memory.
Long story short, the 512 meg card will be faster.
But, you can always overclock the slower card yourself if you still feel you have to have a gig of memory.
note: the 4850 1gb core is oc'd to 700mhz while its memory was downclocked to 1980.
assuming the 9800gtx+ will use the extra 512mb memory the same way the 4850 did, i think the 1gb is fruitful for you.
and considering you want to play crysis, which already consumes 800+mb @ 1680x1050 (not sure if its RAM or VRAM, but its the one displayed once your run the gpu benchmark) @ very high details. i think it would help.
you might want to test GTA4 in the future, that 1gb of GDDR will not be wasted.
and yes you can OC it to bring it close to the overclocked version. 38mhz+ on the coreclock is easy but those of the memory is a little tricky.
remember these factors when oc'ing:
1. your GPU heatsink
2. your warranty
3. calculate the increase in fps relative to the increase in clock (both core and memory). or, if you after oc'ing the game still dont feel fast or it felt no change at all better go back to stock.