It's not so much a question of what to buy but how to set it up. I agree that a desktop is a better option, but if you have genuine reasons for using a laptop for your recording (as I do - portability being the main one), then go for it. You need to decide whether to go 32 or 64 bit for the OS (Win7, presumably), but there's not much reason to stick with 32 bit these days. Ideally the laptop will be dedicated to recording and only occasionally go online (with the firewall on!) for updates and so on. When it's offline, turn the firewall off and run as few services/background programs as possible, as well as a minimal version of the OS with bells and whistles disabled (ask if you need to know how to do this). You won't need AV software if you're careful and this factor helps a lot with speed.
Running a bare bones OS offline, any modern laptop should record just fine. Using your day-to-day computer for recording can be a real hassle, though. Personally, I'd rather buy two used PCs and have one for regular web stuff and the other for recording than have one up-to-date machine and try to do everything with it. Regular day-to-day computing and recording are very different jobs and it's definitely better to do this with two different machines. As an analogy, imagine trying to adapt your family saloon car for specialised duty as a fork lift truck - you get what I mean?
I'm still using a Dell Inspiron Intel Core Duo, 3-4GB of RAM with (bare bones) XP and everything works like lightning, even when doing several tracks simultaneously with various VSTis and other plug-ins. I did upgrade the HDD to a 7,200 RPM one just to be on the safe side, and my audio gets recorded to an external HDD (USB2). The interface is a Presonus Firebox. Most of that is 4-5 years old tech now (yep, still on 32 bit here, although I'm running Reaper which is 64 bit internal architecture anyway - it's not really a huge issue, tbh). This system works fine because I did my homework on how to get it all working as efficiently as possible, which sort of takes me back to my first comment.