Alienware do build okay machines, but yes, I think they are overpriced, and any one that doesn't know about computers, might get them to build the best available, for ALOT of money, when for instance, they may only want to light game, and so are being cheated without them knowing it.
Dell and that, I'd say no to straight away. With laptops, maybe, but not desktops.
If I had no time to build a desktop though, and wanted a quick result, I'd probably pick my parts at a local computer store.
For me, it'd be MicroDirect.
They'd most likely build the PC for me, based on my specifications, which would be just as good, yet probably done faster, than I could do myself.
If there is a shop that'll do that for you, and won't overcharge, etc for doing it, and dump useless software on it, I advise it greatly.
Maybe you can even watch them build it so you know how its done.
I'D RATHER YOU NOT:
I'd rather you not spend loads of money though on an overspecced PC which is going to do more than you want it to.
You said you play Sims 2 alot. Well, even a mid range PC can play that at high resoloutions and settings.
So, I advise you don't spend too much money. Buy a PC with good parts, yes, but not overkill.
The only reason Im saying this, is, because, it doesn't seem like you know alot about PC's, so I'm just saying, going for the BEST out there, isn't always the best option right now.
Especially the 8800GTX and all that in video spec, which AlienWare may put in. A good 7000 series card like the 7900GS (Costs around £155 UK), will do you nicely, for now.
When Direct X 10 kicks off, which the 8800GTX (Which costs £500!) is all about, and there is a cheaper range that'll play the games (as we don't know how good the 8800GTX will perform in Direct X 10 yet, because there are simply no games to test it out on) then it may be worth changing.
Fitting cards isn't that hard, so if you learnt to do that, you'd get the most out of your PC, and your wallet would'nt feel crushed either.
I hope this helps