CPU/mobo:
The i5-750 has plenty of life in it still. With a decent cooler you can overclock it to ~3.4 GHz easily. The LGA1156 socket on the P55 motherboards is a dead-end, yes, but so is any socket in any motherboard you can buy now in a couple of years. So unless you're planning another upgrade within a year, the motherboard is just as good as any other.
So it all comes down to the price. If the i5-750/P55 combo is significantly cheaper than an equivalent LGA1155 or LGA1150 build, I'd say go for it. The difference in CPU speeds even to the newest Haswells is not that significant, especially after the overclock, if your main use is gaming.
GPU:
I would definitely suggest a better GPU if you plan on doing any serious gaming. Have a look at either GTX 660 Ti, GTX 760 or even the GTX 690 (if the latter is still available) for reasonably priced yet fast GPUs. Then again, even a second-hand Radeon HD5850 or a GTX 650 Ti Boost would be much faster than the 9800GT, and cost $140 or less.