also consider a 4890 ,as it is more powerful than a 5770 but lacks dx11 support...most of the current games will perform better on a 4890 than 5770 but if you are concerned about graphics rather than performance then you should go for a 5770
In order to really give you an informed, accurate answer, that will keep you from throwing money away into something that may or may not do a thing for your friend, we need to know your entire system specs to make a good suggestion.
We can only make off the wall guesses without the entire system specs, as Maziar has already asked.
Which 8800GTS, the g80(320-640meg), or the g92(512meg)
There is quite a difference. If it is a g92, a card in this price range is only going to be marginally better. If he has one of these, he might want to save a little more and get a card in the next range up. One of the these cards is going to leave him feeling like he spent a lot of money for very little return.
If it is a g80, I would go with one of the suggestions, depending on what you want, that would be decent upgrade.
It also depends on what processor he is running. A video card upgrade might not do a thing at all for him if he is running an older Core 2, or a AMD X2 processor.
Will a 5770 HD help boost the graphical performance, mostly hes looking to up the graphics in wow to higher settings than what his 8800GTS allows. Should the 5770HD allow for a decent graphics increase?
Also does the 5770 require a better PSU than a 8800gts, or should it be able to use the same PSU the 8800GTS was using with no problems.
You still didn't say what 8800GTS you are talking about. An HD5770 will be much, much better than an 8800GTS 320 or 640mb and about 30% faster than an 8800GTS 512mb. It also uses less power than any of those cards.
Definetly, 5770 is the best you can get for the price point.
It performs great upto 1920x1200
It supports DirectX11
Shader Model 5.0
Its a new card and will be good for more than couple of years or so.
Since your resolution is 1680x1050
It will perform exceptionally in more than most of the game (except CRYSIS)
If you have a PSU of at least 400W, you can run a 5770 on it.
As, new generation cards are extremely power efficient.
You'll feel a significant performance boost upgrading from 8800 GTS to 5770 HD