For that much dough I'd say you won't have a problem getting a sweet box. Most people will tell you about online site which have good deals, but I'd like to toss in a point for buying it from your local store. The prices there are generally (ok, almost always) a bit more expensive, but the extra money is definately worth it. Reason is that, if anything goes wrong with the computer, then it will be MUCH easier to get it taken care of at the local place. In other words, you get back in support what you loose in extra money.
Of course, boxing up things and sending them back to E-bay is always fun too :P
One way to do useful research is to check into computer trade shows. Most metropolitan areas have them, there's usually an entry fee, but it's a good way to become aware of -- and familiar with -- the local dealer pool. Talking to vendors at a trade show is a good way of developing relationships that may well get you better prices down the road, if you ever want to add on or upgrade.
Depending on where you live (and oh, how I wish forum members everywhere would at least give city & state locations in their profiles), you should find trade-show listings in the classified section of your local newspaper(s), usually under the heading "Computers for sale" or similar.