The stupid folks at dell should never have sold a laptop with only 1 gig when running Vista!
I bought an HP with 3gigs of ram and it has issues. Not so much with games but with simple tasks. It really hates playing any kind of video and internet explorer for some reason.
I seriously doubt your warranty won’t let you install RAM. If it actually does, install it anyway, then just take it back out before sending it in to Dell for repair if and when it breaks. The RAM is not really going to harm the computer, so they’d be out of line voiding your warranty for it.
They did it because it probably could run the leanest form of Vista (therefore, technically vista-compatible.)...