this summer, i bought a Hp Pavilion Elite HPE-400 desktop for a fairly reasonable price, and figured, "hey my xbox has been giving me a pretty hard time lately, lets get into PC gaming" with very prior knowledge what so ever, and to be realistic, not very much now. At first i tried to play most MM0's and the frame rate was extremely low and the graphics presenting at the equivalent of 240p. Now i went into help from both my tech teacher and a couple of friends and they both have said look into a upgrade from the ATI Radeon 5450 to a 6000 series. now, i have looked into the system requirements for both the 6950 and the 6990, and my motherboard (N-Alvorix-RS880-uATX) does have a PCI express x16 slot, but will need a powersupply with more wattage (only has 300w, not even sure if thats up to legal standards

). Can anyone tell me if this would work, approximatly how much money would i have to spend (i have a budget of around 450) and if generous enough, possibly send me the links from newegg that would fit between my budget, i would really appreciate it.
Btw, sorry if anyone gets annoyed at a console gamer trying to get into the PC community, Im new to both the forums and fairly new around PC's and just dont want to spend more money on something that dosen't work very well. thanks.