With the games playable on a system that age, DDR2 doesnt make much of a difference; and even if it did, the upgrade to DDR800 wouldnt make more than (perhaps), a 1% difference.
My daughter plays Roblox and Project Cars on her DDR2 system without any issues, and Project Cars runs fast enough to keep her happy.
her system;
ASRock G31m-S
Xeon E5450 (3GHz)
4GB DDR2 800MHz
Asus R7265 GFX (2GB - for some reason I thought it was 4GB when I posted the spec last time).
KingSpan 60GB SSD
1TB Hitachi HDD
Corsair H45 AIO
Aerocool DS Cube in Else Blue.