Err... I googled the specs of a Dimension 4500 and it states that the board only has 2 ram banks total. You said you have 2 slots populated giving 512 (256 *2) so that means if you want 1GB of memory you would need to buy two 512MB sticks. Hope you realized that!

If thats the case I wouldnt worry too much about the voltage issues. You are going to wind up buying a matched pair or RAM, so there wouldn't be a conflict.

Anyway, I find it hard to believe that the RAM you listed is actually 3.3v. DDR RAM's specification limits the voltage to 2.5. I can't even think of a RAM manufacturer (currently) whose sticks can take that voltage for long with out letting the smoke out. But like I said, I wouldnt worry about it.

Browsing name brand memory I found that PC2100 RAM is more expensive than PC3200 (by ~$10 a stick). Running PC3200 ram on a board that only supports PC2100 should be ok, but I can't guarantee it will work with your Dell.