Potential Percentage Difference between PCs.

Hey, first post, seeing as I need this issue addressed, just wanted to ask: How much will the increase in percentage be in regards to the performance?

This is my current rig:
AMD Athlon II X2 245 @ 2.9 Ghz (stock)
(Integrated) ATI Radeon HD 4200
(Unknown brand) 4GB DDR3 RAM @ 1333 Mhz
Asus M4A785TD-V EVO
Western Digital Caviar Blue 500GB
Octigen 400w PSU
Octigen Pro 396 Midi Tower Case

Yeah, I know, pretty bog standard office stuff-  

I'm hoping this is my next rig:
Intel Core i5 3570k @ 3.4 Ghz (stock, no oc)
AMD Radeon HD 7850 2GB GDDR5 @ 860MHz/4800MHz (stock, no oc) (this is the reference card)
Corsair Vengeance 8GB RAM @ 1600 Mhz
Asus P8Z77-V LX
Western Digital Caviar Black 1TB @ 6.0 GB /s
Corsair 650w TX Modular
CoolerMaster Storm Enforcer  

I'm purchasing from online PC dealer: DINOPC - They seem pretty good, above average and the like...  

Also, could someone tell me what temperatures I'm likely to get on my gpu, cpu and inside the case in general... and what're normal temperatures?
I'm slightly new to these things, but I catch on quickly enough.
And where would the best place be to put my Computer Tower/Base Unit?
And finally, could someone recommend a good screen in the sub-£200 area? I'll be using both the new tower and screen for heavy gaming, watching movies and encoding videos

Thanks a lot!