I dont know much about HP, but I've got a dell pc and laptop. One thing to be careful of with dell is the price/spec changes constantly and if you do your research and bide your time, can get an excellent deal. This forum posts dell deals click here If you buy via quidco click here you can get a further 5% off.
With my pc I bought the extended warranty. IMHO a complete waste of money. The folks on here offer a better service. I did'nt bother when buying the laptop.
Hope that this is of some help