C521 PSU & Upgrade Options

I've owned a C521 for approximately three years and I finally decided to give it a major overhaul.

What I am worried about is whether the tiny, odd shaped PSU (rated at 280w) will be able to handle my new hardware.

After some research on this forum and other user comments, I've seen people saying this PSU is actually underrated and performs more like a 300 - 350 w.

However, I am fairly skeptical about such claims.

Does anyone know if there is any PSU out there that will fit in this weird BTX case? 

Yesterday, I made my first step toward my overhaul plan.

I swapped out the old Radeon 1300 Pro for a Galaxy 240 GT Low Profile. (SF IV benchmarks went from E to A instantly)

The next piece would be an ASUS Xonar D1 or DX soundcard (depending on availability)

I also plan to upgrade to 4 sticks of 1GB DDRII 800 RAM, and replace the puny 160GB harddrive with a Western Digital Green Power Series Drive.

I might swap out the DVD burner for a Bluray combo as well.

Would this work? Thanks.