Unfortunately, with Dells-- they usually have only 2
RAM slots-- and with DDR
RAM you should upgrade in pairs... in other words
2- 256MB for 512MB total
2- 512MB for 1024MB total
and so on. Personally I would go for the 1024MB option-- this machine takes up to 2 gigs (2048MB) max-- but that's more than most need... depends on the budget.
Crucial lists 1024 MB kit for this PC for $128.99