If you had £1000 why would you go second hand? Anyway PC for me, OS is irrelevant at this stage.
You want to spend your money on decent hardware the Intel i5 chips are meant to be very good, then you need a decent amount of memory 12gb ddr3 perhaps? Of course if you want to do some overclocking then a decent mobo, psu and heatsink.
Dont forget graphics card, hard drives, 120GB SSD for OS and a couple of 2TB drives for data? Oh dont forget dvd burner and maybe a card reader.
Or you can buy a pre-built machine which is already overclocked... for £1000 you can get a beast of a machine! Unless your a Mac guy.
I would suggest that 12Gb Ram is a bit OTT for most people. I have 10Gb FB-Dimms in my Mac Pro and after a particularly intense day I looked at the stats..... I never actually went of 6Gb so my upgrade from 6Gb to 10Gb was a waste of money
Remember that a few years ago photography was considered an intense use for a computer. The base line for computers has gone up MASSIVELY in comparison to the actual requirement from photographers. With this in mind a normal everyday computer has the power to run CS5 so long as the OS is not sapping it all.
I can run CS4 on my son's 22" iMac G4 without ANY issues. It has 4Gb Ram and a reasonable 500Gb HD. The OS is barely ticking over and there is no virus or other protection required. The only thing that drags it to a halt is entering LIQUIFY but it only means a pause of 4 seconds or so.
I can run CS5 on my studio's 17" intel coreduo 1.84Ghz 2Gb 160Gb machine and again it works fine as does lightroom 3
My MacPro Quad Xeon 3GHz 10Gb 2.5Tb also works fine but despite probably being 6 times as faster as the others there is no real advantage...
My suggestion would be to spend more money on a decent monitor, keyboard, mouse and even tablet as these are the things that you interact with and they make the experience better.
So 27" iMac with it's wonderful screen and base level spec is MORE than enough
As would a £350 base unit plus the rest spent on the bits you see.
The simple matter is that we all fall into the trap of thinking we need more when we don't !! W7 and MacOS are both more than capable OSes we need to spend the money in the right places. You don't need 12 cores and 12Gb Ram for CS5 spend the money on your monitor!!