Hi BlackMoral,
the r9 270 GPU needs a PCIe 3.0x16 Slot that is capable of 15754 MB/s. Your board only has a PCIe 1.1x16 Slot that can transfer about 4000MB/s. The card does not need the whole PCIe3.0 potential so the performance will not suffer a lot, but you can at least expect some decrease in performance just from the board connection alone.
The PSU will be more than enough to power the new GPU, at least if it's not some kind of cheap china PSU that does not output the 550W thats printend on it.
Your CPU is the biggest Problem. I had an Athlon X2 5000+ paired with a GTX660Ti before (Should be almost the same, your cpu is a little bit faster and your choosen gpu a little bit slower). The performance suffered a lot by the CPU (Games ran at 5-10 fps in some cases like Thief or Battlefield in low Settings). The Same GPU paired with an A10 5800+ (Still not a good CPU) was doing just fine in Ultra Settings in both Games.
The only direction i could recommend to you, would be to get a new CPU/Mainboard/RAM with the GPU. To upgrade an old DualCore is not worth it anymore.