Meh I suppose that the backwards compatibility will sort things, if 4 sticks go in then it may be ok. But as you say, mixing modules of different speeds isn't wise.

Though this does raise a question... DELL?!?!?!

Why oh why would you buy a pre-built machine?

Should be ok at PC4200 - spd programming for that speed is 4-4-4-12 no matter what memory you get. Its only when you get to higher speeds that timings vary really.

I think its worth spending a couple of extra quid and having memory you can take with you if you upgrade. Granted DDR3 is on its way but I reckon high speed DDR2 will still be useable for a good 2 or 3 years yet.

The extra 512mb is probably not worth leaving in having said that. 2gb with windows XP is plenty unless you have loads of shit running in your system tray, and if you are struggling with 2gb an extra 512mb won't help matters noticeably. Id just remove whats in there now and install the new stuff. You can always run the newer memory above pc4200 speed anyway using the memory dividers in your bios. You will definitely have the 667mhz option and may have the 800mhz option too.

shame you dont need a stick of 512mb DDR or 4 1GB sticks of DDR2 667...

guess ill never know when ill need em anyhow

If it works with my computer, I'll have it. =)