yeah you can flash a 6850 to 6870 bios, but the memory they use is different so all things being equal you can overclock the 6870 higher and it will out perform the 6850. I think it's worth the extra 20 euro if your buddy can pull it in the budget. I was thinking about going with a 6850 and flashing the bios and overclocking to get 6870 speeds, but I just shelled out for the 6870. I was waffling with an AMD build over an Intel build to save some cash, but you really do notice the speed of the i5 2500k. Plus overclocking is a breeze, I've got mine at 4.5ghz on stock cooling, and was able to take it up to 5.1 without much difficulty. And even if you're not going to overclock yourself, they have 'turbo boost' which takes it up to like 4ghz or 4.2ghz under load depending on how many cores are active and all that jazz which is a noticeable speed increase.
I bought this 6870 Scan.co.uk: MSI HD 6870 Twin Frozr II ATI - AMD Radeon Graphics Card - 1GB - MSI R6870 Twin Frozr II
and it's running at 40C under 100% load in an NZXT tempest case, with pretty standard air flow configuration. I'm not sure how the other 6870's deal with heat, (they are a more efficient version of the 5870 series) but if heat might be a factor, definitely shell out a couple more bucks and you could even get some more overclocking out of it if he notices that he's getting FPS lag and doesn't want to turn down the settings
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