What's the bottleneck in my system?

[Fledge]

i5 2500k @4.7
5870 (XFX stock clocked)
4GB 1600mhz G.Skill

I know that my real bottleneck is my WDCB640GB HDD but I'm talking about in terms of gaming performance - I'm fine with my system being a little slower (not using an SSD) in general as long as I get the frames in the games.

I think my weakest link is probably the GPU so my real question is how much of a bottleneck is it? Would I see a ridiculous performance increase with a better GPU?
Aside from games, would I see any improvement with another 4GB of RAM?

My advice to someone in my position would be to grab yourself a 7000 series card asap if you really want BF3 on ultra and ubersampling in The Witcher 2. I'm slowly forgetting why I even made this thread in the first place. Christ, it must be the man flu.

Thanks all

 

[Fledge]

I was thinking about getting a second 5870, but I believe I may encounter problems with heat as 5870s seem to be pretty toasty. With this system I get 50+fps on BF3 on High @1920x1080. Also I like the idea of a new card entirely - I can always crossfire that in future if I feel I have the funding Currently I believe I need more performance as The Witcher 2 gives me <40fps which is not as smooth as I'd like and it isn't on max settings either. The reason I mention these two games is that I'm playing them both at the moment.

Out of interest, is my 700W PSU capable of handling another 5870? (It's an OCZ ModXStream).
I can't really afford an SSD quite yet and, as I stated earlier, the fps and eye-candy mean a lot more to me than overall speed.

Thanks for the replies