You can skimp on a case, but you get what you pay for which probably means poor ventilation, poor construction and cheap noisy fans should you skimp.
You can skimp on the graphics card, you really don't need to spend much more than £100 on a graphics card to play todays games at adequate frame rates at 1080P. Gforce GTS 250 1 Gig (DX10 card) for example.
Don't skimp on CPU, buy the most expensive you can afford, but remember CPU dictates what motherboard you use and what variation of socket.
CPU cooler (HSF), depends what you are going to be doing and if you are over clocking it, again buy the best you can afford.
Memory you can skimp on, depending how much ram you want and speed it's set at but it is dictated by mobo type.
PSU, don't skimp on, that's a important piece.
Hard Drives you can skimp on but you won't appreciate a slow one.
DVDRW's drives you can skimp on, pick one up for £15.