Every XBox 360 save game takes up memory. Here is a list of the games we've reviewed along with their save game sizes, for you to plan out your memory card purchases. Remember that each memory card holds 64mb, while the hard drive holds 20 gig. There are 1,000k in 1mb. There are 1,000mb in one gig.
Call of Duty 2 - 1mb (1,000kb)
Condemned Criminal Origins - 700kb
Kameo - 100kb
Need for Speed Most Wanted - 125kb
Quake 4 - 5mb (5,000kb)
Perfect Dark - 100kb
Peter Jackson's King Kong - 128kb
Project Gotham Racing - 350kb
So, for example, if you saved 12 save files for Quake 4 onto a memory card, that would take 60mb of space, leaving only 4mb left for other uses. On the other hand, if you were only saving the tiny Perfect Dark save files on the card, you could save 640 of them (640 x 100kb = 64,000kb, or 64mb).
To be honest, we pretty much never use the memory card on our XBox 360. This would only be necessary if your hard drive was dying and you were trying to move the files over manually with a memory card for some reason.
XBox 360 Memory Unit Review