Can I install an 8800gt and 450w Psu into this computer?

First, I'd get a better PSU. 600w or better.

a 500w is usually recommended for a 8800GT, and your computer being a media center has a lot more components added to it. I would play it safe, and make a one time purchase of 550-600 if you can afford it. This way you have plently of power, and your PSU will blow cool air instead of warm or hott. Also if you want to get a 9 series graphics card some day, you already meet the power requirements. =)

Make sure the combined amps are above 30 on 12v as well.

+12V@33A,-12V@0.8A that is from the one you listed.. it should work, but barely.

The difference in price for a better one shouldn't be much either.. maybe $20-$40, which I'm about to save you, and offer you a better graphics card solution.

http://www.tigerdirect.com/applications/SearchTool...

I recently bought this card, and absolutely love it! I previously owned an EVGA 7600gt, with the paddle fan heatsink. I had to clean it every so often, and it got to 80c, which is pretty darn high.

typically a 8800GT runs 70-80c under load. Mine never gets above the 60's, and usually stays in the 50's with most games. This amounts to longer life, and more overclocking capabilities.

The main difference between the XFX Zalman edition and the evga you listed is the size. The XFX will take up 2 slots, because the fan and heatsink stick down further. I would strongly suggest getting a biggest fan and heatsink though, these 8 series throw off a lot of heat. I can't imagine the single slot paddle fan versions. They must get 80c on a regular basis.

Whoever else bought an 8800GT with paddle fans, please comment. You know you're jealous of this deal! $129 for one of the best brands, and an aftermarket zalman cooler! It really doesn't get any better for an 8800GT.

If the graphics card will clear in your computer case, you're fine.

No, your computer will just use the power it needs. Nothing will burn up! Check your Quick Install Instructions, that come with your graphics card. I always put the CD in the computer FIRST, and load the drivers, etc. Then I install the graphics card. Sometimes it's not a prob, if you do it the other way around, but if it is, you have to go through a bunch of changes. A lot of times if the graphics card is installed, then the CD with the drivers on it, Windows 'sees' the graphics card first. Windows then tries to match one of it's generic graphics drivers to the card. Then the graphics cards drivers are 'seen', and a conflict results. Windows: "Which driver do I use?" Your graphics card won't operate right. Needs the driver/s designed for it, by it's manufacturer. (Of course I know you won't forget to un-plug your computer from it's AC source of electricity, before reaching inside. Also don't forget to de-static yourself before reaching inside. Static silently 'kills' computer parts. Just touch the metal frame of the computer case, to relieve yourself of static.

Edit: For $50 more bucks I would look into this vid card:

http://www.tigerdirect.com/applications/SearchTool...

Why? http://www.youtube.com/watch?v=BkUJYKnsgd4

Yep but get a power supply like 600 watt to get your video card to work at full performance other wise there is no point on investing so much money.

These will all work fine.