Your motherboard will support both the 2x4GB ram as long as your Windows is a 64bit version. Also, it has a maximum speed of 1333MHz. Therefore, you will notice a slight lost in performance if your 8GB kit it at 1600MHz or above, this would not matter much, since the performance increase is not quite noticeable in daily usage.
Your motherboard supports PCI-E 2.0 at x16 speed, so a 550Ti is definitely a viable option.
Have fun upgrading your PC. ^_^