2 year old 500w PSU enough to power a phenom ii 955 O.C. + 4890 ?

What power supply? There are some strong ones that might be able to do it, but there are alot of over rated ones that wont stand a chance.

What is the brand of the PSU? Is it generic?

Need PSU brand and specs. But I am going to go out on a limb and assume that, because the OP didn't mention brand, it's a generic.

In which case, the answer is - probably not.

if it only has one 6 pin, then the brand which make it know that it can't hold up a dual 6 pin powered device.

also the minimum PSU for a 4890 is a good 550w one and yours would be about 450w by now.

Ultra, I bought it from tigerdirect.
If i do buy a new 500w generic one would that work?

NO, do not buy generic power supplies, for a 550 watt, which is what a 4890 requires, we are only talking ~$80
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
A generic 500 watt wont come anywhere near that one, hell i have seen some 750 watt power supplies that arent capable of matching that corsair. Pay the money, get a good power supply, avoid having issues and having to buy a good one later on because the cheap one crapped out. Its cheaper to buy right once than buy wrong multiple times.

Really, the power supply is the last thing you should be skimping cash on. REALLY.

well if i am gonna buy GOOD PSU
shouldn't i buy one that can power 2 4890?

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

A good psu that is capable of powering 2 4890s for 80 after MIR

agreed just be sure you purchase at least 1 4890 that comes with 2 molex to 6pin adaptors (I think thats what they are called) since the 650tx only has 2 x pcie connectors.

I'm pretty sure the 650tx could handle a 4890 crossfire, but if that makes you nervous, here:

http://www.newegg.com/Product/Product.aspx?Item=N82E168...
750tx. 4 pcie connectors. Satisfied?