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Try this:
 
PCPartPicker part list / Price breakdown by merchant   CPU: AMD Athlon X4 860K 3.7GHz Quad-Core Processor  (£53.83 @ Scan.co.uk)  CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler  (£23.54 @ CCL Computers)  Motherboard: MSI A88XM-E35 Micro ATX FM2+ Motherboard  (£37.80 @ Amazon UK)  Memory: Kingston Fury White Series 8GB (1 x 8GB) DDR3-1333 Memory  (£54.99 @ Amazon UK)  Storage: Sandisk Solid State Drive 128GB 2.5" Solid State Drive  (£44.01 @ Ebuyer)  Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive  (£39.59 @ Amazon UK)  Video Card: XFX Radeon R9 270 2GB Double Dissipation Video Card  (£105.62 @ Scan.co.uk)  Case: Cooler Master N200 MicroATX Mid Tower Case  (£28.98 @ Ebuyer)  Power Supply: Corsair CX 430W 80+ Bronze Certified Semi-Modular ATX Power Supply  (£37.33 @ Amazon UK)  Operating System: Microsoft Windows 8.1 (OEM) (64-bit)  (£72.35 @ Aria PC)  Total: £498.04 Prices include shipping, taxes, and discounts when available Generated by PCPartPicker 2014-11-23 20:04 GMT+0000

1333Mhz and 1600Mhz don't peform that differently, so really you should go for what fits your budget.
The 860K is newer technology which generally is better than the older technology. It's also less than the 760K for some reason.
 
There are two reasons I chose 1x8GB over 2X4GB, the first being that since the motherboard only has two slots, future expansion will be possible without the need to ditch your working 2X4GB kit. The second is that it is cheaper than 2X4GB, making the £500 budget possible with the 270. The difference between dual channel and single channel is not big enough for a noticeable difference in day to day use either.

I would go for a Xeon E3 Series as you can get them with the same specs as an i7 for under £200 (Minus the IGPU and overclocking capabilities)
The Xeon E3-1230 v3 is a 4C 8T CPU like the i7. That only costs £191.95 over here in the UK. The cheapest i7 from the same retailer costs £227.66.
Considering the previously mentioned CPU is not Haswell Refresh it will work in the majority of boards without the need for a BIOS update.
The Gigabyte motherboards are better to use as they are more common, so if you find a problem you are more likely to find an answer.
Depending on what your friend plans to do in OS X You may find better use from the G3258, but keep in mind OS X is a productivity>gaming OS.
You will also get updates quicker and earlier than an AMD platform because a kernel doesn't need to be created/ported to the latest OS.

If they keep their current but replace the Motherboard/CPU to a Gigabyte Motherboard and an Intel CPU they should be fine.

The laptop has gone off to its client now, but it didn't look like anything more than 7mm, but don't quote me on that.

The things you get with the alphacool kit makes it seem like a better option, for less money. With the alphacool kit you get the liquid and compression fittings, which wins for me over the xspc kit.

Try downloading optifine if you haven't already, then enable Multi-Core on one of the settings pages.
 
 http://optifine.net/downloads
 
EDIT: I run a GT 640 with an 8320 and it runs very well