An upgrade sounds good. What do these look like? How much should I be paying, for a cheap price?

CPU:
AMD Phenom II X6 1055T Black Edition 2.8GHz (Socket AM3) £45 +
AMD Phenom II x6 1055T 2.8GHz 6MB Hex Core AM3 125W Processors CPU HDT55TFBK6DGR £52.68 from Hong Kong
AMD Phenom II X6 1055T 2.8 GHz Six Core (HDT55TWFK6DGR) 95W CPU w/Grease £53.34 from South Korea
AMD Phenom II X6 1090T - 3.2 GHz Six Core (HDT90ZFBGRBOX) Processor £50 + <<< looks promising?
AMD Phenom II X6 1045T 2.7 GHz HDT45TWFK6DGR 667 MHz CPU Processor AM3  £56.91 from China
AMD Phenom II X6 1100T Black Edition 3.3 GHz 6 Core Processor with Heatsink Fan £55 +


GPU:
Sapphire Radeon HD 7870 (2048 MB) Graphics Card £55 +
Asus R9 270 2GB DC2OC 2GD5 £67 + <<< PCI Express 3.0? Will anything above will have its performance slashed to the bandwidth maximum of the MSI 770 C45's PCIe v2.x

Monitor:
tbc

~I live next to Eclipse Computers (Coventry) if it's easier to just pop in there. I'd rather get this sorted before/over the weekend if I can, but will wait if needed.

The R9 270 is a great card, can't really go wrong with that. The only thing I don't like much about your overall spec is the AMD chip, I'd probably swap for an Intel even if it costs a bit more.

It would be nice to find out if the monitor is decent!

My Monitor was delivered (to the UK) in about 5 working days overall. The main delay was Parcel Force, although you can speed them up a bit with a telephone call to pay import charges (£63.55). The box came wrapped in bubble wrap, and I had no exterior damage to the packaging.

I purchased from eBay seller: Dream-seller for £202.90 - with a 1 year warranty.

Colour banding seems good. I’ve check via this simple gradient test: http://www.lagom.nl/lcd-test/gradient.php.  As I understand it, all monitors display very slight, subtle banding, but the worst offenders will appear blocky, and immediately recognizable. Mine is definitely not blocky in any way.

I have no black light bleed.

No dead pixels, although I’ve heard that this tool: www.jscreenfix.com can get rid of some, if needed. I used this to test: http://jasonfarrell.com/misc/deadpixeltest.php

The monitor build quality is best classed as ‘ok’ or ‘acceptable’, whilst the stand is rubbish and very wobbly if it gets knocked. Moreover, the clips at the top of my screen have somehow warped (under heat pressure?). I had considered returning it for this, but I otherwise seem to have a decent copy and TBH I’m not bothered. 

To overclock, I used ATI Pixel Clock Patcher (ToastyX) v1.3.4 to patch my drivers and Custom Resolution Utility (CRU)  v1.2.1 to ‘only’ change the refresh rate. It’s now running comfortably at 96Hz, which is confirmed by the www.testufo.com overclocking test. I quickly tried to jump to 120Hz but get dotted artefacts on the left of the screen.

I also bought this for £8 - DVI-D 24+1 Dual Link Male to Male Digital Video Cable Gold Plated with Ferrite Core: http://www.amazon.co.uk/gp/product/B00OZHGVSY?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00

What's the benefit of running at substantially higher hz?

Here you go: