XFX HD 4850 1GB GPU!

Hi, I just had a few questions for experienced GPU overclockers.

But first the specs:
Setup:
-Sidewinder 2 ATX case
-PSU: 400w NXZT PF400 (12v rail@20.5a)
-Mobo: Biostar A785GE
CPU: AMD Phenom II x2 550 BE (OCed to 3.6 Ghz@1.375v)
RAM: 2GB XMS2 Corsair DDR2 800mhz
HDD: 200 Gb 5400 RPM
Optical devices: 1 DVD/CD burner (DL)
OS: Windows XP Professional 32 bit running on SP3

System goes around 20c-40c, CPU= 30c-44c~ Idle/Load
Basically I am considering Overclocking my GPU, however I have a few concerns which I can find no data on. Also I understand that my PSU is under the minimal power requirements of the GPU. However when I inputted my setup into the "Power supply" calculator, it came out at 350w~ under 90% load.

1. What is a safe overclock for the XFX HD4850 1gb? Keep in mind I only have 3 internal fans, GPU/CPU/1 80mm front for channeling air in. I hate fans... If keeping it stock is the best option thats fine.
Stock Clock: 625 Mhz
Memory Clock: 1990 MHz

2. Would my 400w PSU handle it an overclock?

3. Would my PSU commit sepukku running this setup, if so can anyone give an estimate?


Thanks in advance! Go go ATI HD 5870!!!