unfortunately i have budget for 1 card nd have batman aa so if i take HD i wont be able to play that game with physyx,so i was thinking to go for a low card and upgrade to fermi, but fermi is getting bad previews on the net, like its powerhungry etc.
Well Batman: AA is one game. Some bonus effects in one game that you will likely finish in a few days shouldn't be the deciding factor on a video card imo.
That said both card are good choices at their price point with the 9800GT being a bit more powerful(10ish%) and the HD5670 offering great power efficiency and DX11. However in the US at least you can get an HD4850 for $100 and it's a much better card than either of those.
I haven't checked to see what card is more powerful, and I'm not going to. But you should really just get the most powerful card you can with your budget and ignore all of the DX11/energy efficiency/PhysX stuff.
I really doubt that you are going to be playing games in DX11 with a weaker card like the 5670. You would definitely have to turn down some details, especially if you are playing at a sizable resolution. If I was you, I would rather just play in DX10 with the details cranked back up.
I don't know too much about PhysX performance impacts, but I am guessing that the situation will be very similar in regards to DX11. Your card will not be powerful enough to handle PhysX without turning down some settings. If it was me, I would rather play with the settings cranked back up. So, just like DX11 with the 5670, PhysX with the 9800GT should be discarded from the list of features you take into account
Energy efficiency? As long as the card isn't adding sizable increases to your electricity bill, I would say screw efficiency. At the end of the day, you want the fastest card you can get for your money.
I'm assuming based on some of these other posts that the 9800GT is faster, so I would say get the 9800GT. If they were both equal, I would definitely get the 5670. If the 5670 is more powerful, well you get the point.
It might also be a good idea to check out the overclocking headroom for each card and how they perform overclocked. The 5670 might be able to close some of those performance gaps (looking at the charts posted).
DX11 with the HD 5670 is academic - is just too weak for it. You will get better performance from Batman AA with the 9800 GT. In general the 9800 GT is a tiny bit faster as well. If you are in the US this is a pretty good deal after rebate: