Oh wowzers! So I take it it's a decent card then...
Going to grab it and that psu for now. Is it worth selling my current box ad building a new machine? What do you think I could get for it? 5k?
Depends what you want to do, do you want to do a full new build or scavenge off your old stuff?
You won't get anywhere near 5k for that box. Maybe on Gumtree, but I doubt it, you'd be looking at ~2k if you include the 1TB drive.
Which I wouldn't.
You've got a decent enough base system to get going from.
What I would do is:
GPU
PSU
CASE (I doubt that card is going to fit in your current case)
Then
CPU Cooler (decent one so you can use it on future builds)
You can then OC your chip a bit to get extra performance.
At it's current clock it'll be holding your system back a bit.
Then
MOBO + CPU + RAM
In a PC build there are 3-4 things that can last you for a number of builds that's why I also try to get guys to get
1) Decent PSU
2) Decent case for cooling, expansion size and well style

3) CPU Cooler. These are compatible with most if not all sockets and normally have an additional clip that can be purchased should a new socket come out that it doesn't fit.
4) Hard drives
So in your case you have a nice 1TB and decent enough chip which with a small OC would give you time to save for a new chip MOBO and RAM.