I would upgrade the GPU (go for the 6870) and the DDR to 8GB, i wouldn't bother with an SSD but rather save the cash and use the cash from selling the other bits and think about upgrading Processor/Mobo in the future.

As Clock'd says the GPU shouldn't be the bottleneck at the moment.

I assume you are using windows 64bit, otherwise there is little point in adding memory above 4GB unless you use the ram disk program. Unless your computer is using HD as memory there is no need to upgrade.

The SSD will gain you a little bit speed, but only while loading. If you don't notice any real loading lag then don't bother. I use a 240GB SSD system beside an older one with an older WD2500 drive. The main difference is in initial loading. Once a program is running there is very little between the two in apparent performance. Price per GB is going to continue to drop on these so the longer you wait the more you should be able to buy.

Graphics to 6870 or Gti 550 Ti is a good option. That should enable you to run almost any game at 1080p, although you may need lower than maximum settings in order to be fully playable on more modern games. TBH the difference in Aion between a Gti 550 TI and 7850HD is not that much (550 running on DX9).

Otherwise I would consider starting to saving for a future CPU/motherboard upgrade.

The SSD isn't just for loading, Photoshop uses the HDD as a scratch disk regardless of memory settings - he will see a noticeable performance improvement working on large image files by using an SSD.