Depends on your budget. I am not sure if FL studio is multi-threaded, if it is, I would go for a quad core if your budget allows. You'll have 4 cores, but generally dollar for dollar quad cores will be lower clock frequency. If FL isn't multi-threaded, then go for a high frequency dual core. Check the cache size on the processor as well. You'll have to do some research to find the best $:speed. At some point you will pay 30% more money for about 5% more performance, so look for that line.
A ton of fast RAM, and don't buy the RAM from the company you buy the laptop from, they always over charge. Check the number of RAM slots as well. DDR3 is pretty common, but it's triple channel, so using 2 slots is not going to be taking full advantage. Also it has to be matched sets, usually you can buy kits.