Here's the guide on replacing your card: iMac Intel 20" EMC 2133 and 2210 Video Card Replacement
The RAM cam be upgraded to 6 GB PC2-6400 DDR2 Min. RAM Speed: 800 MHz
A faster hard drive can be installed or a SSD drive. Your optical drive can be removed, an adapter put in for a second internal hard drive and the optical put in an external case. Using TechTools Pro the hard drive can be defragmented and volume optimized.