Well, Alienware is rather high end, and if you wanted to go that route I'd say Falcon Northwest which has a better reputation for price/performance (if you live in the states). I wouldn't go Dell unless you get one of those stupid cheap deals you read about on FatWallet I really couldn't tell you who to buy from, your best bet's to put it together yourself. As mentioned, you'll get cheaped out somewhere pretty much no matter who you buy from. It really is pretty simple to put a system together, there are boards set up to explain how with a bunch of guys wanting to demonstrate their l33t knowledge on the subject.
If you're looking for a gaming rig, AMD is the better price/performance. For office and multimedia, Intel hands down. The important changes that are occurring right now are not CPU related, but are video and PCI. Athlon just released their 64-bit CPU, and Intel just released the Prescott. It's definitely worth waiting for a few months.
Consider what it'd take to get your current system to last longer. Usually a new video card would be the answer to rejuvinate a system, but like I said it's not the best time for that.