I have upgraded the ram and added a bigger video card on my computer. I am also thinking of replacing the 40 GB hard drive to a 160 GB. I read on TigerDirect that the power supply may need to be upgraded as well. It is currently running on a 250 watt ps. Do you recommend this additional upgrade and can it be done by a novice? Also, can this small ps be causing my system to freeze up after about 5 or 10 min. of running fine? It freezes every time. Sometimes it won't go past the start screen (or blue windows logo).
Any help is greatly appreciated!

If you've replaced your RAM and graphics card you should know the inside of the PC..the power supply is quite easy to replace.

http://www.fonerbooks.com/r_power.htm