Having been down this route and although being able to hook up a GPS to the laptop, it isn't really that practical. Laptops can become very big, which is why a lot of people leave them in the office/at home. You invariably end up with a laptop, a big power supply, a GPS receiver that also needs powering and the impracticality of having to turn sideways to see where you are. I found my own solution and it works well - but there are cheaper solutions too.
My own answer is a handheld computer that can sit above the dash in its own powered mount. That mount (With its own speaker for directions)also powers the separate GPS rcvr, a Fortuna Clip on. The Software chosen for me, is Copilot 6 Live. This way, the PDA does much that the laptop does, in addition to SatNav but without the bulk, power and space requirements. In fact, I begin to wonder whether I should flog the laptop. PDA, Mount, GPS receiver, software and Mount weigh in at about £500 but there are also PDAs that are, albeit slower, more biased towards SatNav and already include the GPS unit and software. These can be bought for around the £200 mark.
Recent mobile phone development now features SatNav on many models and with some is available for upgrade of existing models too. There are benefits here too, in that the telephone package will include the connectivity to fully use the features withiin current software, such as traffic jam info, sending new routes to you, etc. etc. Worth a look and especially if someone else is paying your phone bill.
Finally, the standalone SatNav unit such as Tom Tom is well worth looking at. (Although you can also get Tom Tom with many of the phones mentioned in previous para.)
So - It is a big World, but the key to success as ever, is simply having a good idea of what you want it to do for you.
Hope it helps,
Conan