---[EDIT/UPDATE]--- Well I was typing this up while you guys posted those last two comments, I never considered changing the case since that wasnt the question asked but it is a good solution to the problem I guess. Well, I hope I didnt just do all those hours of searching and researching for nothing.
But it sure looks like I did, now Im sad
Alright _Aries_ ,
I think I have this all figured out for you now. Far as I know the PSU in you PC is called a TFX form factor or Mini ITX IDK, as I suspected but I didnt want to throw out any guesses, and snipergod87 also suggested it could be TFX.
So I put your PC specs into
Antec Power Supply Calculator and came up with 263w to power you PC, thats with the new GPU. I gave you the link to the calculator so you can check this yourself also.
When it comes to the TFX or Mini ITX form factor PSU's they seem to not exceed a 300w limit, so I have found this one:
SeaSonic SS-300TFX Bronze 300W on Newegg.com that should fit your needs as long as its the right for factor and not Mini ITX instead, but IDK, and you dont add anymore componets to your PC, that is after your video card of course. Seasonic is the TOP manufacture of PSU's so you do not have to worry about quality, it also is a 80+ bronze rated PSU so you will have good power efficiency.
As for your video card, just to say it again, I recommend you get this, unless as mosox says its low-profile then forget about this card:
XFX HD-567X-ZNF3 Radeon HD 5670 1GB You have unitl Nov. 30th to still qualify for the MIR.
Well I guess that does it, I hope that answers the questions you had. If you have any other concerns about what I have covered just let me know.