First off, what size memory card do you have/use?
I have 3, One is a 259 block, and then an AC 59 block (came with AC) and a 59 block (its larger as its not an AC one).
Are any full?
The Animal Crossing one is full completely with AC, NES, and Sonic Adventure DX emblems. (total: 59)
The regular 59 block is somewhat empty, only around 10 blocks used.
259 one has about 120 left on it.
Do you keep saves from EVERY game?
I delete most files after I beat them, like James Bond and Need for Speed (except Nightfire and Most Wanted). Some I keep just in case I liked it, like Most Wanted, Underground, Pikmin (1 and 2), Paper Mario, Sunshine just to name a few. I do keep some if I know Im going to play them hopefully again like Rayman 3, Melee, Driven, and Ocarina of Time (Collector's Edition) and Mario Kart to name some.
If I had an 1012 or 2024 one, I'd keep every save probably.