What is my setups bottleneck?

Hi everybody this is my first post and it should be a relatively simple one. I would like to know what you think is the weakest link/bottleneck in my set up (Excluding my PSU as I will be upgrading the wattage).

My set up is as follows:

-Phenom x4 II 965 OC'd to 4ghz (stable)
-G-Skill 6GB (3x2GB) DDR3 1333Mhz Ripjaws Memory Kit CL8 (8-8-8-21) 1.5V *
-Sapphire Radeon HD5850 extreme 1024mb DDR5
-MSI 870A-G54 Socket AM3 8 Channel Audio ATX Motherboard
-WD 500GB 3.5" SATA-III 6Gb/s Caviar Black Hard Drive - 7200rpm 32MB Cache
-XFX 450W Core Edition Pro PSU

*My after market cpu heat sink blocks my 1st RAM slot so I have one stick spare at the moment so potentially 8GB RAM with a different cpu fan. I use n Arctic Cooling Freezer 7 Pro rev 2 and my max CPU temps when gaming go no higher than 51c (32-38 idle)

I will be looking to upgrade at some point this year and would like to see what other people think should be my first choice in hardware changes. You do not have to recommend any hardware to me, but if you feel you want to then please do. I am not a beginner when it comes to hardware, but would like to see others opinions on what could be improved before I invest.

I read that I should give some information on what I will be using my PC for in the sticky so here is a quick brief.

My PC is primarily for hardcore MMO gaming as well as internet browsing. The current MMO I play is "TERA" and I would like to make changes to increase my FPS. At the moment the highest I can get my FPS to is around 50-57 with it usually idling around 30-45. I play in 1920 x 1080 and manually set my graphics options to give reasonable performance (which still looks pretty nice by the way).

So, that should be enough information. If you require anything else please do ask.

Any comments/ discussion will be extremely appreciated.

Thanks.