Im thinking of either selling or upgrading my current system, wich is:
Core i5 750 @ 2,7 Ghz + Coolermaster Hyper 212 Plus
Asus GeForce 560ti
Intel DH57DD motherboard
Western Digital Caviar Blue 500GB harddrive
Samsung Spinpoint F1 1TB harddrive
Kingstion HyperX blu - 8GB @ 1333Mhz Ram
NZXT LexaS Case
Coolermaster GX 750W
Depends. how much money are you wanting to use to build a new system. The system you have is decent. You could upgrade to a new Ivy Bridge i5 or i7 and maybe get a better power supply. ( not a fan of coolermaster for PSUs) After a CPU upgrade you could get a 2nd GPU. Again its up to you how much you want to put into a new build. You can always just wait until you see what you want for a new build, and borrow parts from this build to complete it and save money. Then you can just sell off the parts individually.
If you already have a $800 buyer then I would sell and start over. Just don't sell it to a friend who is then going to pester you about supporting the system for the rest of your life. Otherwise i would keep everything and upgrade the mobo, CPU, and possibly GPU when the 600 series comes out.
Your rig is not really outdated, so I would be hesitant to say that you would get much more performance from a new rig at this point in time. Sure your CPU could be faster, but you can OC. Sure your GPU isn't a monster... but you could SLi for cheap. And your ram is not the fastest, but 1600 is not much faster, and it is not likely the bottleneck in this situation.
Knowing what games you play, what quality you play at, what res you play at, and what you currently get in FPS vs what you would like to get would make our suggestions easier to give.
Oh, forgot to mention Ivy bridge is right around the corner. I don't normally suggest waiting for an upgrade on a new build, but as you already have a good system it will not kill you to wait a month or 2 to get the extra 10% performance bump over Sandy Bridge.
Im really thinking to sell and get these parts:
ASRock Z68 Extreme3 Gen3 with an i5 2500k
some corsair psu 750w i think ill go for
1600mhz kingston ram
optical drive dvd writer
mechanical storage hdd and a ssd
some kind of nice case
and then run on integrated grapichs until the 600 series is out
for those parts i will be $ 822
What do you think? =)