I want to upgrade all my hardware, but also want to keep my microATX case because I like the way it looks, easy to carry and I have some good modding ideas for it. My problem is that I want to purchase a K7VM4 (http://www.asrockamerica.com/Products/K7VM4.htm) and pop in a Athlon XP 2800+ (that i might overclock a bit). The highest microATX PSU I can find is a 300W. I also want to use a a G4 Ti4200, 2 HDDs, about 5 80-92mm fans, 512 DDR, and later, a PCI GFX card so I could use dual monitors. I need to know if the 300W can support that much hardware.

nope. Just buy a 420 or 450 and save the other one for a spare.

Yeah, you can replace the PSU and keep the case.

Do you mean replace the PSU with the 300W microATX PSU or with a 400W ATX PSU? I may just buy a miniATX case so I could use a 400W PSU.

You may have cooling problems if you do that. Less space, more heat. I would suggest finding a case that comes with the power supply you need. That way, hopefully, someone has given some thought to cooling and addressed it somehow.

Cooling won't be a problem at all. I am just wondering if 300W is enough for a slightly OCed 2800+, G4 Ti4200, 2 HDDs, and the periffials.