Underpowered PC? HELP!

Guys, I need your help.
I've been running a P4 2.8 recently, with a RADEON9800 SE and 3 Hard Drives (60GB,120GB,200GB). New mobo as well.
Long story short, the 200GB started experiencing problems, so fearing a crash, I backed everything up and replaced it with a 250GB. Things are worse now, the 250GB refuses to be detected some most of the times in the BIOS, and sometimes I get a message saying that my GFX card isn't plugged in.
I'm on a 300Watt PSU. Do you gys think this might be causing the problems? Any help is much appreciated.

I'd guess your diagnosis is correct, you've got quite a lot of power-sucking hardware in there. Try disconnecting two of the hard drives and see if the system stabilises; if it does that's a strong indication that you need a bigger PSU to run all that stuff.