thesize micro atx your reffering to is either the case of the mobo. the size all psu's are the same[ or very, very close]. they are designed to be interchangable and work for most anything. the older psu s however do not have pentium 4 support [the 4 prong yellow & black wire connecror]. these psu s do not work with the new pentium 4's but if you have an amd or celleron d, etc. you do not need it. there is a few psu size conflicts that i don't think you need to worry about, in my case i can't get about 3 of the old big [450-600w] psu's in because of whatever reason the size somthing or other. but i have a costom case and i dought u have a propritary case. you should be good 2 go.. :firebounce
yeah just be carfull apon the install make shur you take note of everywhere that needs power as you unplug the old stuff and of course watch out for static