What to do with aging components (5 years old) and tight budget.

For the pentium 4 pc, I would upgrade the motherboard, cpu and ram to the ones you have there for yourself, if you can bump it to an i5 or at least an i3 though you would benefit from it.

The both the hp and core 2 quad build have decent cpus still (hp has c2d from what I coulkd find out), so don't spend money on upgrading the cpus. What I would do with them though is upgrade to at least 2GB ram in both of them, 4GB if your budget allows. AS photoshop benefits from RAM bandwidth, make sure that you run 2 evenly sized RAM modules as then you use dual channel and double your RAM bandwidth.

In the hp I would also get a cheapish gpu to help with opencl in photoshop and general usage, something like an HD 7750/7730 or 6670 would still be fine on you 250w power supply. If the price of this is going to be about the same as upgrading to a Celeron G1620 though the cpu (again an i3 if your budget allows) would be a better investement as the system as a whole would benefit more.

Things to check on the builds where you will be replacing the motherboards are that the case uses standard ATX or mATX mounting holes, displayed here: http://images.anandtech.com/reviews/cases/DTX/interoperability.jpg otherwise you will have to buy a new case also.

Lastly make sure the power supplies are use a standard ATX layout. This is probably only going to be something to check on the P4 build as the two newer should use it, but it's better to be safe. ATX power supplies have a 24pin connector which attaches to the motherboard. If they have this connector, chances are it's an ATX power supply though do a little research first as there are a few pre-built computers which used standrad ATX connectors with non-standard pin layout. If it has this non-standard layout it could damage or break ytour new motherboard.

EDIT: Do not waste money on water cooling for overclocking, or any sort of cooling to help your overclocking. On such a limited budget you would be better off investing that money in higher quality, faster components. For businesses also stability is crucial, so the possible instability caused by any but the most mild overclock can lead to bnig headaches down the road due to lost time.

I don't know much about the rest of the things you need to know but hopefully that can start you somewhere.


EDIT2: looking at the HP again, it says you cannotuse more than 2 1GB mosules in in, so you are limited to 2GB total on that system. If you can afford it, I would upgrade that system to something a bit newer and have 4GB RAM