PC slowed down after upgrading the PSU

A Psu doesn't make a computer faster or slower. It just alow it to work. So you must look for an answer about that decrease in performance elsewhere.
And yes, that PSU has power enough to handle a Phenom.
About the power distribution, nowadays a typical PC pulls more power from the +12 V outputs because the microprocessor and the graphic card, which are the two most power hunger components from the PC, are connected to the + 12 V outputs.
Your old PSU can deliver more power from its +5 V and +3.3 V outputs because the design of that PSU is outdated. Some years ago the power distribution was different and the PSUs were made in order to work properly with that hardware (the old cpus weren't connected to the + 12 V outputs).

ok, there is something i seem to be missing here, isn't the +3.3V is the 4/8 pin ATX cable ?? or the new PSUs supply +12v through the 4/8 pin ? because i see new MB having the same 4/8 PIN.

another thing i noticed, most new Mother boards have 8 pin ATX connector for the CPU, the OCZ have only 4 PIN, will it work with new motherboards, this is supposed to be designed to the new power distribution, it should have had 8 PIN.

of course i understand that the PSU won't affect the raw performance of a machine, wht i was talkin about is some lag, the PC seems to freez for a couple of seconds when under heavy load, this never happened with the old PSU, so i assumed it has to do with the new one, plz correct me if i'm wrong.

One last question, isn't the 152W from the +3.3V and the +5V enough to handle a mother board and a 86w processor ?

1) The ATX12V connector is a 4-pin connector which is used to provide current to the CPU.
The EPS12V connector is a 8-pin connector which has the same target of ATX12V. Since it has 8pins instead of 4, it is capable of providing more current. Not all power supplies/motherboards have it.
2) You can plug the PSU's ATX12V connector on the motherboard's EPS12V connector, however this isn’t recommended. But you can do it.
As I have write before, your old generic PSU was based on the very first ATX specification, which was created when computer power consumption was concentrated on the +5V line. Nowadays, power consumption is concentrated on the +12 V outputs, as the CPU (ATX12V and EPS12V) and graphic cards.
3) A low quality PSU (or even a defective PSU) explains some stability problems as computer freezing or computer rebooting by itself because of the high noise/ripple level (outputs aren’t perfectly continuous) even when the computer isn’t pulling all its power. It can be caused by a bad work in the filtering stage.
4) Wattage is not everything. You must pay attention on how clean the outputs are.
Anyway, The OCZ StealthXStream 500 W should be enough. The +5 V output is produced by two rectifiers connected in parallel, each one with a maximum current limit of 30A giving us a maximum current of 152W.
This FSP power supply is not a bad PSU (it even presents low noise and ripple levels).
Maybe you could have a defective one. If I were you, I would contact OCZ to explain them what's happening with their PSU.

a different psu shouldnt decrease the speed of a pc.

i am not fond of ocz psus that much but yours is plenty good, even for a phenom setup.

I know everyone says no, but I was told by a hardware engineer many years ago that he observed a measurable system slowdown caused by an underpowered machine. I doubt its the problem here, but I have never been able to completely rule it out.

your power supply doesn have enough juice on the 3.3v and 5v line to power your old system the newer atx power supplies provide more juice to the 12v lines the older power suppplies had a lot more on the 3.3v and 5v lines, zippy emacs are some of the newer power supplies with a beefy 3.3v and 5v lines