Even if you knock the price of software and the other bits off my estimate, it's still £350-400 a machine.
Most people who use internet cafes aren't going to be bothered about Quad core. So long as there are computers with all the software they could need, they'll be happy.
I doubt the profit margin in internet cafes are that large, so you want to spending as little as possible on the actual machines, because the rest (like you said, consumables, wiring and software) will soon add up.
There's no point going the whole hog without getting some basic usage down first. Spend as little as possible on the basic machines, and then run a customer survey to find out if they want to game on the PCs. The Vostro 200s will take another 1GB of ram and a half decent graphics card no problem, which will be pennies once you're getting a constant revenue.
However, I doubt you'll need to do that, since most people go to an internet cafe to catch up on email/messenger, school work, research and browse the net. The dual core is more than enough for that. No point spending more than you have to. You could offer printer/photocopying services. However, that'd be something to add down the line unless you can afford it now.
Edit: You might also want to look at this:
http://www.ncomputing.com/internet-cafes.aspx
Could be a good alternative to buying 6 machines, a server, etc.