[ati radeon hd 4870] ddr3 vs ddr5

It will be slightly slower than the GDDR5 version.

how much is the difference in fps.

What kind of a question is this?
It will depend on the type of game, rest of the system, graphical settings and many other factors.

It should perform like a 4850. The 4850 and the 4870 share most of the same everything, but the 4870 has faster clocks and DDR5. Seeing as they changed the memory, make sure they didn't change the clocks as well.

Well from my experience, there shouldn't be much difference in FPS value, but DDR5 have higher load speed of games and in stuff like rendering the scene and all. At least that what I experienced.

Out of the topic, but HD4870 is a little bit old, much heat and power consumption.
You'll better with newer tech like 5xxx or 6xxx series.

I would get Sapphire HD 5770 Vapor-x or something like that. Or gtx460 if u can afford it.

no my uncle already bought the ddr3 version of the 4870...now my problem is the power supply...what should i ask for when i buy a new psu..

Well, any 500W PSU from trusted brand like Corsair, Antec, OCZ, Seasonic, PCP&C, SilverStone are more than enough to run single HD4870.
I know they're expensive, but you will not regret it later, those are high quality PSU.