They either use standard supplies...for full size supplies..or they use micro atx...which is still "standard" but just the smalelr standard....usually on HP's it is a micro atx.
I sold a compaq 600z presario about a year ago..it seemed to be all standard and had an athlon and AGP slot...and a full size supply.
You can also get upgraded supplies now for micro atx stuff..like this one
http://www.newegg.com/app/ViewProduc...103-448&depa=0 ...great for upgrading/repairing those pavillion/emachine/gateway comps
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The main thing nowadays is just to make sure you have an agp slot