Trying to build a computer

Well for just casual gaming you will be fine with this setup.
The AMD cpu seems good and the motherboard is fine.
-----thats my opinion-----
wait for others to review and see what they think.
However if later down the line u want to keep gaming at good resolutions casually in like a year or so you will have ot upgrade graphics card to gtx series from nvidia or ati 5000 series which ever suites u

I actually put the wrong graphics card in there this is the one i wanted
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Everything on your list is compatible. I think you can do better for the money though. For example, the 9800GT and the 250GB hard drive are a bit older in hardware terms. Going from your list, it looks like your budget is ~$700, is that correct? Hang on a bit and we'll see if we can get you a better setup for the same price.

Edit- The HD5770 is a much better choice.  

Yes im trying for something under $700 but what do you mean the hard drive is a bit older like are there better hard drives for about the same price or are there just ones that are faster?

Yes. Both.

Like what hard drive would you recommend for about the same price?

The fastest mainstream hard drives will be the ones that pack the most data into the smallest area (platter density). Right now, one of the most popular hard drives is the Samsung F3 500GB. All 500GB is on a single platter = good thing.

No one has supplied a list yet? Well, I'll give it a try:

AMD Phenom II X4 965 Black Edition 3.4GHz
Sapphire HD5770 1GB GDDR5 Video Card
Asus M4A79XTD EVO AM3 790X Motherboard
Combo = $414

Mushkin Enhanced Blackline 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory $98

SAMSUNG Spinpoint F3 HD502HJ 500GB 7200 RPM SATA 3.0Gb/s 3.5" Internal Hard Drive -Bare Drive $55

ASUS DRW-24B1ST/BLK/B/AS Black SATA 24X DVD Burner - Bulk - OEM $20

ASUS VH202T-P Glossy black 20" 5ms Widescreen LCD Monitor $130

Total = $717 before S&H, but also before -$55 in MIR

That will get you the fastest AMD quad available, RAM with low latency, one of the fastest 7200rpm hard drives, an overclock friendly motherboard, the video card you want, and a monitor 2" bigger than your first list... and a basic SATA DVD±RW.

*Note- this list is based of your original post. There isn't a case, PSU, OS, mouse, keyboard, or speakers (well the monitor has speakers but that doesn't count, does it?)

So what kind of PSU would i need to power the computer you suggested?

That's a pretty good parts list.
I'd probably get the PII X4 955 to bring the cost down a little, unless the budget has some wiggle room. It will overclock easily on the stock cooler to be as fast as the 965.
As to the PSU, 500W will give you plenty of headroom, but something like a 430W Earthwatts would be sufficient.
The PSU is not the place to go cheap though. Look for full range active PFC (no little voltage switch) and 80+ certification for efficiency. Antec, Corsair, Seasonic, and Enermax are among the better brands.

Thanks for the answer but would this PSU work with the parts i wanted to buy? http://www.amazon.com/Corsair-CMPSU-550VX-550-Watt-Cert...
Also is the computer in rwpritchett's reply good for a decent gaming computer like better than the one i made up? and if so would that PSU power that one?

The Corsair 550VX is a top quality PSU and will work for your proposed builds. It will blow your $700 budget though. Decide what your absolute budget is and that will decide where to cut back. You could probably drop down to an Athlon II X3 440 or Athlon II X4 640 to make room to budget for the PSU. Or you can decide to stretch the budget a bit to keep the high clock Phenom.

The HD5770 is an excellent mid-range card. If you want to do more research, read the latest Tom's monthly article:

Best Graphics Cards For The Money: July 2010