Graphics card compatibility

Get the "Upgrading and Servicing Guide" here> Manuals for HP Pavilion Elite HPE-400f Desktop PC - HP Customer Care (United States - English)

That case is pretty tight but the HD6950 should fit ok, if your using the stock power supply plan on upgrading it also with the video card > Newegg.com - CORSAIR Enthusiast Series TX750 V2 750W ATX12V v2.31/ EPS12V v2.92 80 PLUS BRONZE Certified Active PFC High Performance Power Supply

is that power adapter necessary? is there one i could get for a better price?

Quality in a power supply is important, it's not an area you should skimp on, cheap supplies go bad and burn out other components in the PC like video cards and motherboards.
http://www.techsupportforum.com/f210/power-supply-information-and-selection-192217.html

well its not that i dont want to spend the money, its just that i only have about 400 in the bank after the card. My brothers paying for half, but i dont think hell agree to another 50 dollars. what's the bare minimum i can go with little risk? Besides, i'm not planning anything really extreme, just mainly source games. Is there a risk of waiting to get a new one for a few months if i don't do anything insane?

Drop the Video card down to a HD6850 and use a 650tx v2.

thats not much cheaper, so i think i will see if my brother can pay for half, and see how that works.