What is the difference between them and what is a graphic memory (i.e 256, 512) ?
They mean the same really, sometimes gpu refers to a video output that is apart of the motherboard. Video cards are pluged into a slot on the motherboard, typically the PCIx16 slot on modern MB’s. Video cards can have as little as 64MB of RAM on them, good ones for gaming can have 512MB and up to 1GB. The amount of RAM is not an indicator of performance, but the type of RAM on a video card can. Cards with GDDR3, GDDR4 and GDDR5 are the best today and to buy a new card one would want to avoid the older GDDR2 and GDDR1. Older video cards should be avoided since they generally use more power, and newer models have power saving modes as well. Their are tons of reviews on the net, just type the model number or a new card like 5870 (gaming card) and the word ‘review’ along with it.