Your post caught my attention because I still have an XPS-630 (among other computers). Just a few thoughts about your question:
- You probably already know this, but unless your OS is 64-bit, it can't make use of more than your current 4GB of memory.
- The increased performance from the increase in memory may not be that perceptible (depending, of course, on your uses).
- Memory for the old motherboard is probably going to be more expensive than memory for a new motherboard upgrade.
- Your $100 bucks might be better used toward an upgrade--motherboard, CPU, memory. The 630i case is kind of a classic, so many have upgraded. See my630i.com for more info and help.