I bought a new laptop end of January, cost me £400 in a sale (currently £600 I believe) and i play in top spec graphics and have not had a single problem with lag, unless it's been a jägex problem or my internet connect which also has it's moments of spackyness!
All I know is its a HP laptop, didn't spend a fortune, and there's nothing particularly special about it either. I think pretty much all NEW laptops will do the job for rs now