Help upgrading dell 8400 video card/cpu

Hi everyone I've tried posting on other sites without much luck, this is my first post.

here's the specs of my desktop pc:

dell dimension 8400 2003? 2004?

3.0ghz pentium 4 630 pcu

4gb (4x1) DDR2 533 ram

2 hard drives - 150gb for windows xp, 120gb for linux/ubuntu

graphics card: "PCI-E x16 port #2....ATI RADEON X300 (RV370)"

power source is 350-400w, not exactly sure (I also want to upgrade this along with the video card)



I use my computer for everyday things and also like to watch movies, I have a handful of games I play also. I want to spend around 150$ to upgrade just the graphics card (that's how much I want to spend just for the card). I would like to move up from a 128mb, someone recommended somewhere else a NVIDIA gts430 but I'd like to have something a little more than that. I'm keeping this computer for another 4-5 years and want the graphics to be up to par. I would like to get a used psu, what size should I get to go along with a good graphics card and slightly upgraded cpu?
V
V
V
I just bought a dirt cheap 35$ + 5$ shipped, pentium 4 3.8ghz 670 to upgrade. Do I need to run any BIOS updates before installing? What is a good fan/heat-sink to work with the new processor (better than stock) fits in a LGA775 dock. I know it isn't the best computer on earth, but it's better than what I have.

thanks for the help,
Mike