Outlet X140 / 6000 $250 off, and question about RAM

As of about midnight this morning (1/26), Dell starting offering $250 off on outlet X140 / 6000 in a two day sale. They also seemed to have dumped some good deals in the outlet at that time, and I think I'm lucky to have picked one up as a refurb:

XPS M140 Notebook: Intel Pentium M Processor 750 (1.86GHz/2MB Cache/533MHz FSB)

14.1 inch WXGA TrueLife Notebook Screen

512 MB DDR2 SDRAM 533MHz (2 DIMMs)

8X DVD + RW Drive

80 GB EIDE Hard Drive (5400 RPM)

9 Cell Primary Battery

2200 Wireless

XP Home (really don't need ME anyway)

Microsoft Works Suite

$679 before S&H/tax, $753 after. Pretty good deal, so look out for more deals in the next couple days!


Now the question: What makes this less of a deal is that I will want to upgrade the RAM. I was considering keeping 1 DIMM of 256 in the notebook and adding in a 1 gig DIMM of a different brand (Kingston and Corsair are cheaper than what Dell offers). I've heard before that it negatively affects performance to add RAM 1) of different brands and 2) in imbalanced pairs. Is this true? Am I better off putting in the 1 gig and taking out both 256? And are 2 DIMMS of 512 better than 1 DIMM of 1 gig for some reason? I understand that both DIMMS of RAM will need to be 533 to run at 533.

Thanks!