Hello Dex,
Here is the link to the website I refer to for rankings. It is updated often. Prices for some cards are quoted but I suggest you check at Newegg or TigerDirect.
http://www.videocardbenchmark.net/gpu_list.php
In this website, you can click on any of the column headings and arrange the data in ascending / descending or alphabetic / numerical order. Since the Geforce 8800GT has a ranking of 101 try and avoid going below that. The other recommended card by SSE is the Radeon HD 4850 and that has a rank of 63 but you can choose one with a rank of 70 too. The concept is to stay as close as possible to the cards recommended for SSE. Needless to say, your budget also matters. If you do home down on a card, check its power requirements (watts). As your Acer Veriton has a low wattage PSU, the better cards may not be suitable. Don't ever risk popping in a card whose wattage requirements are high. The capacitors of the card will eventually blow and this is not covered under warranty.
Further, do not select any card which has 'Mobile' or 'Mobility' in it. These are for laptops.
vin_sun