Looking to upgrade the following two systems on a 100 budget total. Will need to start saving for a new system.

what i have:

Dell XPS Gen 4
3.2 Ghz Pentium 4
1 gb ddr2 pc4200
Geforce 6800 pci-express 256 mb graphics card

Custom Built
Amd Sempron 3.2 ghz
1.5 gb ddr pc2700 or 3200
HD 3450 AGP 8x Graphics card

I was thinking about increasing total system memory for the dell to 2 gb, and leaving the other as a lost cause. What do you guys think? Will doubling the dell's memory help the game run smoother? Is the custom built a lost cause?