Technically, you still can’t use above 3.2-3.9 GB depending on the system. However, when you enable PAE it allows the 32 bit OS to use memory from 4-8 GB or so. PAE just allows you to “see it”.
Short answer is: if you need more than 3 GB, why aren’t you running 64 bit edition?
PAE has been on all Intel chips based on the P6 and later cores, including all Celerons. PSE was enabled starting with the Pentium II Xeon, all Pentium IIIs, all Pentium IVs, all Pentium D’s, and the entire Core line.
64 bit support is another story entirely.
And I’m surprised it won’t see more than 3 GB – I can understand a memory hole from 3.x-4 GB – that’s pretty normal. But if you install 8 GB you should have access to ~7.2 GB or so.