Replacing the PSU in a Dell Studio Slim 540s

Hi, I bought a Studio Slim 540s back in November 2009 and now I've decided I want to do a little gaming on it. However, it runs a bit hot and it doesn't have sufficient energy for a new card. I think I have a PSU that is at 300w.

I was looking at either a GTX 460 or ATI 5850. The 5850 is probably way too big for the case. So, I was thinking of replacing the case with something simple but effective. I like this:

http://www.newegg.com/Product/Product.aspx?Item=N82E168...


That case should be good enough, with perhaps adding a fan or two. Any recommendations for good cooling fans?

Now, on to my next problem. I'm keeping everything else in the Studio Slim at stock, the Quad Core, 6GB ram, etc. Only thing that is changing is the PSU and Graphics Card. However, I've heard Dell Mobos/PSU are proprietary and that makes the PSU hard to replace. I need around a 400w-500w PSU. What is compatible with the dell Mobo chipset? Help would be appreciated, thanks.

Some of my specs if it helps:

Mobo: Dell 0M017G A00 P45/P43 chipset
CPU: Intel Core 2 Quad Q8300
Ram: Dual Channel 400mhz(DRAM frequency) DDR2 6GB
HDD: Western Digital 7200RPM 32mb Caviar Black
PSU: reads 10A 125v so I'm guessing its at 300w?

I don't think the stock PSU in the Slim is worth risking on a 5850 or a 460. Also, I need to replace the case anyway because the Dell case not only sucks but it runs too warm for my liking. It should be noted that I'm only upgrading these parts for this PC because next year I'll probably be building one from the ground up.

Anyone know any specifics about this Dell mobo?