You'll need at a minimum
Pump (DDC or D5 are popular)
Reservoir (personal preference)
CPU block (EK HF are excellent)
360 (3 x 120mm) radiator
Tubing (several brands to choose from: tygon, xspc, primochill, mkleer etc..)
Compressions or barbs (barbs are cheaper, compressions look better imo)
2 x GPU blocks if cooling the 5870s (EK or XSPC blocks are good if going the full cover route)
I doubt you can get it all brand new within your budget of 300 especially if cooling the 5870s. If silence and cool temps are your thing then its worth cooling the cards but at around 70 quid for a 5870 block it gets crazy expensive. I don't overclock my cards so couldn't comment on that front. I always water cool GPUs now and couldn't imagine running one on air but you have to decide whether you're willing to pay that.
You can get used blocks at auction for less than half the price of new but make sure it comes with all the accessories if you do.