Yes that CPU is still powerfull it will not bottleneck the 560 Ti, the PSU is plenty. Well, if that 4870 just ain't cutting it for you anymore, get an 560 Ti. Oh, and make sure you get the DCII version of the Asus GTX560 Ti, i know guys who oc'ed it to 1100Mhz core stable!
^.^ I still have my 1gb 4870, and I just yesterday swapped out my q9550. Only reason I did a swap was because of some instability, but still a great chip for almost all games. You may need to upgrade your CPU/mobo in a year, though you'll be happy upgrading your graphics.
All the newer cards are still PCI-e 2.0 so yes they will all work with your board.
For the money you are spending on the 560 you might be dissapointed in the performance lift over the 4870. I would try to stick it out with the old card until the full line of AmD 7000s come out. That should drive prices down and give you more options.
Can my motherboard handle the latest graphic cards though??
and i worry about the size of the card, which might be too large for my case?
Currently i'm using a Antec Sonata III
your motherboard will be able to handle current pcie 2.0 cards.
looks like your case dimensions are:
16.7" (H) x 18.2" (D) x 8.1" (W)
42,5cm (H) x 46.3cm (D) x 20.6 cm (W)
dimensions of evga gtx 560ti
9" x 4.38"
If i get the GTX570, can my PSU handle it with all that I have on my M board now?
Your PSU is fine for the 570, however, I would strongly suggest investing in some aftermarket cooling for your CPU and doing some overclocking - the Q9550, whilst still a viable CPU, is aging and will present a bottleneck in some CPU intensive games (Multiplayer BF3 being one example of this). Overclocking can allieviate this bottleneck partly, or entirely given enough time and effort, for a minimal price (the price of a good air cooler).
If you have no intention of overclocking I suggest going for the 560ti - it will be quite the upgrade from a 4870 512mb.
overclocked cards offer slightly higher performance. they consume more power than the reference design (non overclocked) cards. usually overclocked cards have coolers installed on them, but if you want, you can choose to install your own vga cooler.
imo you should be fine with a card with stock settings. better, a gfx card with good cooler.