Dell CPU upgrade, PLEASE HELP!

Hi

I have a Dell Dimension c521 (AM2) PC. It originaly came with an athlon x2 3800 (90nm, 89W), and then I upgraded to Athlon x2 5600 (65nm, 65W) and OC it to 3,2 GHz.

I want to upgrade to a phenom II, but here is the deal:

1)The bios of this specific mainboard hasn’t been upgraded since 2007… it was actually luck that the athlon x2 5600 worked on it.
2) The mainboard supports an AM2 processor (up to 89W)… that’s the reason C521 owners could never upgrade to a Phenom 1, since that first generation of phenoms was >90W….

With some of the new Phenoms II (and athlons II) rated below 89W, I was hoping I got a chance to grab one of these and upgrade my good old PC. Any Sugestion? Thanks!