DDR or DDR2?
I think you should just upgrade your existing motherboard to 1 gig or more of DDR400 because you will notice a pretty big imporovement with just that.
To go over to DDR2 probably requires a new processor too because you need the memory controller that is embedded on most CPUs. And that means a new Heatsink fan and possibly a new power supply. And DDR2 is more expensive.
Compaq Presario SR1720NX desktop computer
AMD Sempron 3500+ AMD64 processor
Microsoft Windows XP Home
AT&T SBC Yahoo DSL Home
A+, Network+ Certified