Here's the service manual for the Dimension 9100. It says DIMMs must be installed in pairs, so I assume that it will come with 2 256MB modules. It has four slots, so I see no reason you couldn't just slap in the two new 512MB ones and have 1.5GB total.

Interesting:

Quote:
NOTE: Memory purchased from Dell is covered under your computer warranty.

So your new memory is warranted longer than the memory you got (and the ones you would have gotten) from Dell.