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I would take the new one just because I know how to put whatever I want on it. And getting to try Vista for free would be good(though a friend of mine already has a pirated version apparently and has been offering everyone a crack at it).
Experiences:
-Networking professor just laughs at it(he majorly dislikes it and doesn't feel that currently it is as productive as XP)
-My friend with the pirated version likes it on desktops but on laptops is more iffy on it since Vista can really drain the resources much more visibly on a laptop than a desktop.
-Various other people I know who have tried it dislike it because of how much it slows down the system.
-Another note; I have heard that the boot loader is not as safe in Vista as it has been in previous versions. This could be a very dangerous issue, though I'll admit that I have yet to try the OS myself and thus can't be sure how easy it is to mess stuff up like that(before I install vista I need a major processor upgrade, which means I need a major motherboard upgrade, which in turn could cause me to acquire all new hardware(except hard-drives) due to older hardware in my system).
Err.... anyways I would just go for the new one, it wouldn't be to hard or long of a process to do a wipe and re-install of XP. And it wouldn't require a lot of waiting(I hate waiting for a computer to be fixed). In addition for what you want to do with it Vista may be just fine.
However, if you do go with XP do remember that up until 2006 Win98 had still been supported. So I doubt they will just drop XP. They'll wait at least a couple years after the release of Vista to discontinue with XP, and if Win98 is any hint it may be more than just a couple years
.(and yes I still have a '98 system and it runs much better(quicker, smoother) than my XP system(and my xp system is on a computer far more powerful than my 98 system), trouble is nothing supports it anymore *cries*, that and 98 can really do some funky stuff with you lol).</span>