Well I'm getting a new computer but dunno what to get and don't know how to customize so was thinking off going one off the Dell Xps models would you recomend these or what what one would you recomend XPST 630 or XPS 420
420 is not worth it. I have one and don't like it because it costs a lot for the chasis and the little Sideshow screen that I have no use for. The best PSU will give you 425W so it's bad for higher-end graphics updates. It's also BTX so no motherboard upgrades either.
630 is better because it uses the ATX layout only its reversed so the motherboard is upside down (it isn't a problem though). If you want to upgrade later on then you can save the case and grab a new motherboard. PSU is about 700W if I can remember (incorrectly probably) so plenty of room for more hard drives or better graphics cards.
Building your own computer is, by quite a lot, the best option. I swear I can build a computer with the same specs as the XPS 420 I have at around $700-$900 while I bought it for $1300. Plus, you get what you want and are not limited to what the maker could supply.