Yea, I did read your other post & Matt gave you some excellent advice. Before you buy a power supply you have to check with HPeee to find out if a store-bought PSU will physically fit in the case & you also need to know if HPeee used proprietary wiring on the stock PSU (& therefore the motherboard). If they did, you either have to buy an HPeee replacement unit or an adapter to convert the standard PSU wiring to the HPeee wiring scheme. If you buy the wrong style PSU, install it & switch it on without checking these things out beforehand, there's a chance you'll fry the entire system.
Here's the the images of the card you selected...take a look at the 3rd pic. See where the 4-pin HDD plug from the PSU needs to be connected?
http://tinyurl.com/lxymt
A decent PSU is gonna cost you $50 or more, so this "upgrade" is gonna cost you approx $200...that's a lot of cash to invest in an outdated system.
Here's the system requirements for FEAR:
http://tinyurl.com/mxqxp
And here's some FEAR benchtest comparisons. The XT version of the 6800 isn't tested but the GS version (which is better) is...see where it ranks on the chart:
http://www.tomshardware.com/2006/05/15/graphics_card_buyers_guide_2006_part3/page44.html