You have 4 computers the same? Are they all having lockup problems? Are they all showing high temps? Or is it just one that you've upgraded recently. If its just one computer, and you've put a p4 in it, then the lockups could be power supply related as easily as overheat related. Pentium 4s require a fair bit of power to make em go that fast, and if your power supply isn't great then your system can have problems.
The first thing you should do (I can't believe I didn't suggest this before) is run the pc with the case off for a while. Take note of the temperatures (both in BIOS and in MBM5) and see if the system feels stable. If your system temps drop a lot (10+ degrees) or if it stops locking up then we've probably found the culprit - lack of cooling.
Anyway, adding two more fans won't strain a power supply - they don't take much juice.  
you'll probably have a spare system fan header on your motherboard to power one of them - that'll take a standard 3pin fan connector. Check your manual first. For the other you'll need a 3pin fto 4pin adaptor like 
this. (I've found SVC to be a pretty good store for cooling).
Before you buy any fans, take the case off and check you've definately got fan mounts  (they generally look like 
this) and what size they are (the norm is 80mm diameter). If you have then you want to buy two fans, an adaptor, and some fan screws (fans don't use standard pc case screws, but annoying unique ones).
Trust me - if you've done any kind of work inside your pc before, you don't need the shop to do it for you - you just screw them in and plug em in and you're done. All you've got to do is make sure they're pointing the right way, and they have arrows on them to tell you 
 
As for which fans - well, the more airflow the better the cooling. But the more airflow, the more the noise. The perfect balance is well nigh impossible! It depends on how close your case is to you, and how sensitive to noise you are, and if you leave your computer on in the room while you sleep. I'm sure everyone here will have tons of suggestions on which types...