Potential Percentage Difference between new and old PC.

I do not know what the percentage will be but it will be a massive upgrade in all aspects. The 3570k blows the Athlon II out of the water and the HD 7850 can game like a champ, it makes the integrated 4200 graphics look like childs play. To sum it up you will be able to do anything you please with your new computer  

That cpu is beautiful and it can stay at temps even below 50C in the right conditions, but anything under 80C is fine.

Keep an eye on it with this - http://www.cpuid.com/softwares/hwmonitor.html

The GPU, anything under 80C is good also. I'm not sure if the reference design comes with temperature monitoring software or not though  

Always make sure there is airflow around all areas of the tower and try to not set it on thick carpet without having a hard flat surface in between.

Enjoy!

Ah! Yes, very well! Thanks for your input... I'll need a few more opinions before I can settle for the specific unit.

By "in the right conditions" in regards to the CPU, what do you mean exactly?
And for temperature monitoring software, can it be downloaded easily?

And can you recommend any monitors for me in the sub-£150 region? Thanks again!

Hi, can you say a bit about what you do with your PC? Ex: For web surfing the machines may be the same. For gaming huge difference.

Re CPU "Core i5 3570k @ 3.4 Ghz (stock, no oc)" Non K version will game the same and cost a good bit less if you do not plan to OC. The price premium is for unlocked multipliers that let people OC.

Right conditions - getting a solid aftermarket cooler + good thermal paste, good air flow inside the case, having reasonable or low room temps, getting a chip that was manufactured at the highest quality with no flaws.

As for the temp software, yes its as simple as clicking download and a quick and easy install. Don't worry, its a safe site as well.

What size monitor? 20-25 inch okay? 1080p required? speakers necessary?

Yeah, I mentioned it above,
"I'll be using both the new tower and screen for heavy gaming, watching movies and encoding videos".
And yes, I plan on overclocking my CPU at a later date which is why I got the 3570 "k".

Hey, thanks for the reply. You've comforted me a great deal.
Oh, sorry for not specifying, I'd like to have a 22" monitor up until a maximum of 25". Yes, I would prefer 1080p (necessity) and speakers aren't necessary. As I've mentioned before, I'll be using this screen to display maxed (hopefully) settings on a few current-gen games like Battlefield 3, Metro 2033 and Crysis 1, 2 (and hopefully 3), so I'll need decent contrast ratios and the like.

Thanks again!