What's your budget?
TL;DR - get the Phenom II 715 linked at the bottom of my post and the Radeon 7770 linked before my cpu info discussion. If it must be slightly cheaper, get the linked Geforce 650; if you have any more cash, go 650ti over the 7770.
The 8800gt was a great card in it's day. It's only DX10, so you aren't able to run WoW in DX11 mode, which does offer a notable performance increase. To wit: http://www.tomshardware.com/reviews/world-of-warcraft-cataclysm-directx-11-performance,2793-7.html
So, just based on that, to grab a DX11 card that has about the raw power of the 8800gt will yield a notable improvement. As I putz around on Anand's comparison tool, it looks to me like you need at least a Radeon 7750 or Geforce 650 to out-muscle the 8800gt. Cards like the Radeon 6670 and Geforce 630/640 aren't really worth it as a true upgrade. A Radeon 7770 or Geforce 650ti would be cake if you can stretch the budget that far. So:
$80-90 / $100 / $110-120
If you go 7750, make sure you get one with DDR5, *not* DDR3. There is a great deal on a 7770 right now at Newegg - $90 after rebate:
If the outlay is too high b/c of the rebate, here's a 7750 for $90, no rebate to mess with: http://pcpartpicker.com/part/his-video-card-h775f1gd
Or a Geforce 650 for $80 after a $20 rebate: http://www.newegg.com/Product/Product.aspx?Item=N82E16814127703&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
650ti cards start at $125 after rebate on Newegg.
Regarding the cpu...I googled your machine: http://support.gateway.com/s/PC/R/1015005R/1015005Rnv.shtml
Here are the motherboard specs: http://support.gateway.com/s/MOTHERBD/Shared/4006254R/4006254Rsp2.shtml
There is no bios update: http://support.gateway.com/us/en/product/default.aspx?partNumber=1015005R
All of that said, the Phenom 9600 is a socket AM2+ cpu at 95w tdp. There exist three Phenom II cpus in AM2+ packaging:
The Phenom II X3 715 @ 2.8ghz: http://www.newegg.com/Product/Product.aspx?Item=N82E16819103679
The Phenom II X4 920 @ 2.8ghz: http://www.newegg.com/Product/Product.aspx?Item=N82E16819103472
The Phenom II X4 940 @ 3ghz: http://www.newegg.com/Product/Product.aspx?Item=N82E16819103471
The 920 and 940 are both 125w tdp cpus, while the 715 is 95w. The 920 and 940 are both over $100 on Ebay right now, and it's probably not worth paying that much for one, especially since the board doesn't explicitly support 125w tdp cpus.
There is an X3 715 on Ebay right now for $60 though, and if it's in your budget to do so, it'd be worth picking up. The seller has a 30 day money back guarantee so it's not even a risk. You'd gain 500mhz, 4mb of L3 cache, and the newer architecture that wasn't gimped like the original Phenom. To give you an idea of the comparison, here is a Phenom 9650 (revised version of your cpu) vs. a Phenom II 720 (AM3) - you'd gain between 18% and 25% fps in games: http://www.anandtech.com/bench/Product/23?vs=83
Here it is:
Edited by Selvaganesh on 3/26/2013 10:04 AM PDT