I think if you've had your first scan and everything was ok then it's fine to start buying stuff. In fact now is the perfect time because there are loads of NCT Nearly New sales on, and you'll pick up some real bargains!
I guess it depends on how large your house is, though. If you live in a rambling pile, then it won't matter if you've got the cot, pram, high chair etc in plenty of time. If your house is smaller, then maybe stick to buying clothes, toiletries, small bits of equipment, bedding etc as you find stuff you like, good deals etc.
Bear in mind that some of the larger pieces of equipment may take a few weeks to arrive, so it's good to plan early. Also if you're one of those people that thinks girls should have pink and boys should have blue, then wait until you find out at the 20 week scan, or at birth, to get the gender-specific items you need.
Search the archives of mumsnet too, there are lots of threads advising the best things to buy for your baby, and stuff that you won't need. That will save you money and space!
Oh, and congratulations!!