Do you have go get 5.1 in one shot? Your budget is very meager to set up the whole configuration in one purchase. I would highly recommend building up in stages (I bought my speakers incrementally over a period of one year!), in the following order.
1) Mains
2) Sub
3) Center
4) Surrounds
Now if this is possible, over time you can put together a much better system when compared to getting a 5.1 setup for $300.
In fact, looking back, from the time of the first purchase (Axiom M60's) to the most recent (Denon 3910) its been 3 years, with various upgrades like Receiver, TV, Universal remote, etc. interspersed in between.