As said above, laptops really aren't suitable for FS unless you buy an absurdly expensive one. Avoid Sony!
I have a desktop that cost under £500 and runs X-Plane (IFR practice) well.
Much of my work these days is done on an IBM ThinkPad T43. It is an excellent laptop - it does all I could ever ask of it - and it cost me £300. Reliability has been 100%. It has a 1.86GHz processor, 1GB of RAM, and a 60GB hard drive I could have had 250GB HD for another £50 but since my previous (5 year old) ThiunkPad R31 was 30GB and not full, I decided I didn't need it.
A friend of mine lashed out serious money on an HP laptop, which started to go wrong after a few months. HP took it back (not without some argument) and fixed it - but the fault is well known and he's afraid it might recur.