HELP Please - Replacing 7600GS 512MB PCIe

Thats not a bad step up from the 76GS.
You really should give more information about your PC and/or what your planning to do with this card. Gaming or non gaming.

What are the specs on your power supply. Make sure your powersupply supports the amperage required for the videocard.

input voltage (47-63Hz),100-127V/8A,200-240V/4A
maximum output wattage - 300W.

That card sucks for gaming. The 8600 series was a flop.

If your budget is around $160, go for this:
http://www.newegg.com/Product/Prod [...] 6814129101

Good PSU for $61:
http://www.buy.com/prod/corsair-vx [...] 66485.html

I don't think that powersupply is enough. You need 16amps on the 12v rail. The 7600gs series dosen't have those kinds of power requirements which is why I went with it.

nV is a step ahead in high end cards ($250+). As far as midrange, ATi offers the same quality/price as nV.

600w would be fine (and overkill for 86GTS/2600XT range card) just make sure you know what the +12 volt rail has for amperage.

MTV is better than VHI, as Coke is to Pepsi - sure Coke came out with Cherry cola first, but eventualy Pepsi caught up...
What I'm saying is it's a... stigma? nVidia breaks the barriers, ATi cleans up the mess. Anyhow, don't worry about the "who's better" because the cards your looking to buy are matched evenly.

As far as the PSU - watch it. Some (few) high wattage PSUs can have crappy +12 volt rails < on which the videocard runs.

The 450w Corsair linked above would be a nice PSU for you.
It should handle what you need it to do.

But you need to upgrade your current PSU to add a better GPU.

I have a micro atx setup and my case uses a regular power supply. Unless HP has a custom setup (can anyone verify this??) When you look at getting a power supply you will see somthing listed like 12V1=18a 12V2=16a in the specifications (newegg.com has a tab which shows the specs) Which means you have 34a between 2 12v rails.
The video cards above the 7600GS series usually have a higher power usage therefore the need for a more powerful powersupply. If you were to just put in a 8800 series card your problems would vary from distorted display, frequent lockups, sudden computer shutoff or the computer may not even power on. In the extreme end it could burn out your existing powersupply.