Upgrading computer for Radeon HD 4850

Hello. I recently ordered and recieved a Radeon HD 4850, without looking at my power supply and under "guidance" of someone. It's a very large card and hardly fits in my case, but that's not the point. According to the instructions, it came with a cable that needed to be plugged from the card to the motherboard/power supply. Nowhere on my motherboard, is there a spot to plug it in. Second, the box says it needs a minimum of a 450w power supply, while my computer only has a 300w one. What is the cheapest way to upgrade and use the 4850? Would it be to buy a barebones and the CPU, PSU, and etc, buy a weak computer and throw the parts I have already in there; or buy a PSU, CPU, RAM, case(may not if I can fit the motherboard in), and motherboard? Specs here:

http://h10025.www1.hp.com/ewfrf/wc [...] &cc=us&pro
duct=1152517

I also upgraded the ram not too long ago to 3 gb, and added in a 200gb hard drive. I'd prefer to use most of the same stuff if possible, like my HDDs, and CD reader/dvd and cd writer but I'd understand about new ram, seeing as most motherboards have switched to DDr2, now.

This is the card I got:
http://www.newegg.com/Product/Prod [...] 6814102824

Thank you in advance.

EDIT:
Scratch all that, I'm a total idiot and just misread the box! I just need a PSU upgrade. If a mod would so kindly delete this post now...

I just went though this exact same scenario. I wound up just buying a new 500w PSU. Here's a link to amazon where I bought the unit:

http://www.amazon.com/Cooling-PPCS [...] n_edpp_url


I have a Dell Inspiron 531, and it fit in there just perfectly. No issues at all.