If you upgrade your graphics card, make it a jump of several tiers or you will be disappointed if you do not see major improvement.
Your 5850 is a good card, but it is only a bit slower that a GTX560ti.
You should be looking at a GTX570 or 6970 at least.
Perhaps even a GTX580 or 6990.
Your 700w psu will run any single card if it is of decent quality.
An oc to your cpu might help also. Try it.
To help clarify your options, run these two tests:
1) Run your games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
2) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 70%.
If your FPS drops significantly, it is an indicator that your cpu is the limiting factor, and a cpu upgrade is in order.
It is possible that both tests are positive, indicating that you have a well balanced system, and both cpu and gpu need to be upgraded to get better gaming FPS.
want to max out crysis 2 dx11, and most other stuff
Unless you have a very high budget to work with, don't bother trying to max out Crysis 2 with DX11 and ultra settings. If you read the recent articles here on Tom's benchmarking Crysis 2's DX11 and ultra upgrades, it indicates you would need at least 2 6950s or 2 6970s in crossfire or equivalent to get smooth gameplay at maximum at 1080p. Crysis 2 has become like its predecessor, it makes all but the most powerful graphics configurations cry.
It's a difference, but not enough to be worth an upgrade. It's worth noting that they're supposed to be competing on the same tier. The 5850 rivaled (and decisively lost) to the GTX 470--which is pretty much the GTX 560Ti. It's like upgrading a 2010 Nissan Altima to a 2010 Toyota Camry. Yeah it's better, but you're not gonna do anything new.
As far as your RAM--I assume that's 2x4GB + 2x2GB? Just run the two 4GB sticks, it'll be quicker--unless you're running a bunch of virtual machines or something. Run Memtest86+ from a USB stick to verify the throughput change.
I agree with everyone above that you'll need a pretty big upgrade to notice a difference. I think you should either buy a second 5850 to get the performance of roughly the GTX 460's in SLI (see iam2thecrowe's link) or...no--that's what you should do. Buy a second 5850. That or throw down $500 for a GTX 580.
Too bad you got an odd P55 board with one 16x and one 4x PCI-E slot. Even so you might want to just try adding a second 5850 for more performance. If you're going to stick with a single card then only a GTX 580, an overclocked 6970, or a dual GPU card would really be considered a measurable upgrade. Powercolor just released a 6870X2 after all. Anyway, it's up to you.
That's why Radeons allow it. I think that PCI-e 1.0 got people hating on the slow 4x speed though (because it was half as fast then), so NVidia doesn't allow it. But I don't think that's to their advantage since people with 16x/4x motherboards now just buy Radeons. Actually, all these restrictions on SLI licensing has hurt NVidia--especially with the majority of AMD CPU customers choosing Radeons for future CF.
Back to the initial 16x/4x comment:
OP, your 16x/4x mobo will run dual 5850's very well (and tessellation with kill them). You'll be more limited by your i5-750 running at stock speeds.
Tessellation--at least in Metro 2033, I don't notice much of a difference with Tessellation On/Off. I mean, I know there's a difference--but is it better? It doesn't make oil drums any rounder (still polygons viewed from top instead of round)--I guess I'll have to wait for a good implementation of it rather than a novelty one.