Just want to echo the thoughts here... $2500 is enough to get you an insanely fast, totally top of the line machine, if you custom build it yourself (or even if you get it custom built by someone else, usually.)
For home video editing, you probably need a Firewire connection (aka an IEEE 1394 port), and you will definitely need a large hard drive (I'd go with at least 120 GB).
For gaming, you'll need a respectable video card. The Geforce 5700 that Lasereth mentioned is fine, and you can get better ones for a little more money, or slightly les powerful ones for a little bet less money. I prefer ATI Radeon video cards to nVidia Geforce, but I determined before that it's because I'm an elitist bastard, not because they're necessarily better. Either one will do you just fine.
For memory, I would get at least 1 gigabyte, and it has to be at least 400 MHz. (Is that PC 3200? I can't remember off hand) There are different speeds of memory, just like there are different speeds of processors... a lot of people don't know that, and they try to beef up their machine by getting a lot of really slow RAM. Many commercial companies do this, too, to save money.
My recommendation would be NOT to buy a commercial PC, because they cost more and are less powerful. The one main point for buying from someone like Dell, however, is that they offer support. If you aren't very comfortable with computers, it IS nice to have someone to fall back on (I mean besides us at TFP
)
If you want it custom built, I'm sure any one of us here can get a good system put together for you (Lasereth already did one possible version). If you're worried about building it yourself, one of us could probably even put it together and ship it to you for less than the cost of a commercial PC. Or I could do it, and you could come to my house in Maine and get it. Heh.
What it really comes down to is whether you'd trust having a custom computer or if you would prefer to have the warranties and service that come with an expensive pre-built machine.
Everyone here will tell you to get a custom built machine. You just need to decide for yourself. If you want a pre-built machine, then we can also give you advice on which brands to look at, and even which specific models to look for. Just say the word, and I'm sure someone will be happy to talk your ears off (like I have) about buying pre built machines. I mean, that's what we're here for, right?
And just a quick question for Lasereth... why would you include that Raptor hard drive? I'm just curious what exactly the effect of it would be on the machine. Would having the OS on the SATA drive make it faster overall, or what?