Some questions for my first build

This is going to be a work in progress, since living on a college student's budget is pretty hard on the wallet...

First Phase:
Mobo: P35 or P45? I haven't really seen a good case for going with the P45 yet, even with its PCI Express 2.0 capability. Any suggestions on brands?
CPU: Either E8400 or Q6600 I was considering the quad since I do a bit of sound and video editing. I also plan on overclocking (first time, should be interesting...)
CPU Cooler: The Xigmatek HDT S1283 seems like a great deal, but any suggestions would be great
Case: Mid-tower, no idea on what to get with all the new cases on the market, I would be up for any suggestions

Stuff I already own:
RAM: 2GB (2x1) A-DATA DDR2 800 (PC2 6400) (Plan on adding 2 more gigs down the road)
PSU: Rosewill RP550V2-D-SL L 550W
HDD: 250GB Seagate Barracuda 7200.9 (Taking from old computer so I can save a bit for now since it has XP already loaded on it)
GPU: XFX nVidia 7900GS (overclocked to 619mhz core, 825mhz memory)

I know that graphics card is ancient, but it will be good enough for another month until (i hope!!!!!) the prices on the 4870 drop

Any feedback would be greatly appreciated!

fist off a budget might help but i will do what i can...

p45 it really isn't that much more and while it isn't that much better now it will be soon enough. brands... i would go with ether ASUS or Gigabyte both
good board makers

personally i like the Q6600 it just more future proof which is a good thing for you because i would guess you would like this rig to last more than a year and a half

ether the xigmatek or the the zalman 9700 led are great if you get the xigmatek make sure to get the retention bracket.

on case choice the Antec three hundered is a steal at $60

That Rosewill PSU may be dicey. Jonnyguru has found some good Rosewills in reviews, but the brand as a whole remains on tier-5 ("Do not buy" ) of a commonly referenced PSU listing.
On the mobo, I'd go with Gigabyte or Abit. Gigabyte techs very publicly called BS on some claims by Asus not long ago, and I'd want to know how that fell out before trusting Asus. Even if some of their products are good, I would prefer to not do business with a company that is dishonest.
As much as I liked my old Zalman 7000-Cu, I would not consider the 9700 after seeing the price of the Xigmatek and reading the reviews.
On the case, I'm currently using a Coolermaster CM-690, and it is roomy and has great cooling. I generally like any of the Coolermaster cases I've used, although I've often had to re-tap some of the mobo mounting holes in them.

Incidentally, moving a hard drive with OS from one computer to another can raise lots of issues with chipset drivers. Even if you get it cleaned up and working, your final result is still not likely to be truly clean, and won't perform as well and may not be as stable as a fresh install. Research this topic before you give up on buying another OS license.

+1 for everything jtt283 said.

You can get an Antec earthwatts 430W psu now from newegg for $30AR shipped. Good deal for a good psu. Should be plenty to power your system. But if you plan a much higher oc on the q6600 or plan to get a hd4870 or gtx280 etc, you'll need a bigger psu.

for the hard drive part here is what you are going to need to do

when you put it in (make sure it is the write kind) you will need to go to the boot order in your bios

change the the order so your your computer boots the copy of xp

from there you can reformate the drive which note will delete everthing on the drive so make sure you have everything you need backed up.

when the formating is done go back to the bios and boot the hard drive and install your copy of windows

hope this helps and good luck