Memory interface

Don't get hung up on the little details when the broad strokes of gaming benchmarks are easy to find.

I believe the lower-end GTX600 series had a slightly constrained memory bus, the primary result which was to cause a notable performance drop above 4xMSAA. On the other hand, most people wouldn't game at over 4xMSAA on lower-end cards.

Long story short is:
a) pick a BUDGET for a graphics card upgrade
b) Google reviews for cards in that budget and compare

I'd probably be recommending the AMD HD7000 series below $200 (HD7770, HD7850, HD7870) depending on your budget.