Let's see... 850USD is about 535GBP, according to Google.
It's not local to you (in the UK, to be precise), but I know of a PC refurbishing place that provides bog-standard PCs for much less than that, and potentially pretty decent spec ones for less than that as well. (Let's see, the obvious 'upper end' of the currently advertised stock is £175 ($277-ish) for a Dual Core 2.8Ghz 2Gb RAM, 250Gb HDD, 17"" LCD, DVD-Re-writer, keyboard and mouse of course. And a fully legitimate Win7 install, including official MS recovery media should you/a future repairer ever need it.) Not sure what video card you might like to augment/replace the decent but perhaps the one inserted (or the on-mobo outlet) is not a
totally up-to-date/gamer-quality one, so add a bit if that's a concern, also you
could double the memory. Otherwise, it's a pretty darn good machine for that price.
For pitfalls to avoid,
do check that you're getting a legitimate licence (what's stuck on the side of your box), as some more amateur refurbishers can cut corners on that (perhaps using a Volume Licensing Key that they shouldn't be, and then when MS gets wind of that and revokes it there's
big problems for
all their customers). If for some reason you think you're forced to return to the same site to reinstall the machine, consider why (although it might just be laziness/simpler that way, and still otherwise legitimate). Check if you have enough in/on the boxed-up package to reinstall from scratch
yourself. (And that this matches the installed OS, whatever that is. I've heard of enough people with Vista-upgraded-to-7 machines where this was done by a "backwoodsman" PC repairer with a handy disc... perhaps 'as a favour', and then when things go wrong a "repair of the OS" ends up being "reinstall from scratch". With files copied off and back, but programmes (including a 'moody' copy of Office) would have to be resourced from their original source media. So, anyway, check that, but use your judgement.
As for building yourself, although as a more experience builder now I know you can be
too scared of static (anti-static straps, the whole kaboodle), of course you would be better safe than sorry if building(/rebuilding) your kit from the ground up. Beyond that, have fun doing it and (barring the more expensive ones you can have, which you should avoid)
learn from your mistakes. Almost always you can redo things (whether that's reinstalling the OS, or switching the memory into a different pairing of slots), but watch out for not properly thermal-compounding the chip/heatsink interface, or causing shorts where you shouldn't do.
Of course, if you fry something
because of what I've just said, it's
not my fault.