Remember, its a DELL. If you buy a new motherboard, you will have to purchase a new case, as the DELL cases and motherboards and specially made for each other, its not standard.
If you buy a new motherboard, get one with a pci-e slot, and get the 7800GT or GTX - fantastic cards!
Yes a jump from 512mb RAM to 1GB RAM is a nice jump, you will notice the difference.
But with DELL's, if you order them with 512MB RAM, if comes as 2x256, in most of them anyway