the more memory, the better it can handle higher resolutions. If you were in the market to buy a new card, only 3 would do.
3850 with 512 mb of memory
3870 in the same flavor
or a 8800 GT with 512 megs of memory.
Your card is far from bad, but these are the current solutions that are most sought after, going above them is pointless, considering these cards offer the performance of their high end 2900XT and 8800 GTX for a faction of the price.