For that sort of money you wont see a lot. If you are doing all the work and dont have to pay for labour, get a couple of exchange reground cams with a fast road profile, you can get a complete fitting kit with a chip to suit from Piper for about £400 I think. Then put a decent element in the airbox a K&N or Pipercross or something will do, say £50, then £250 -£300 will get you a decent stainless cat back exhaust incl a decat pipe. Thats £750.
If youre paying for labour theres not a lot you can do for that sort of money. Chip, filter, zorst are all well and good but without the cams you wont see a really noticaeable difference. Nitrous is a nice option if you want cheap power in short bursts. There are a lot of nitrous specialists around nowadays, you could probably just about do it for £750 fitted, but youd have the smallest bottle which is a bit of a waster of gas. A valver engine in decent nick will easily take a 50 shot.