Laptops in general are not designed for taking apart like desktop machines are and infact if its under warranty you could invalidate by doing so if it has a seal, so take caution there. Yes you can get Ram upgrades for Laptops but quite expensive compared to desktop RAM.
You will get more bang for your buck with desktop without doubt. Laptops for convienence Desktops for sustained performance and upgradability. Yes there are powerful Laptops out there but expensive compared to a equal Desktop.
Don't forget that laptops reduce their processing power as the battery reduces power, so in that sense unless you using from the mains it's performance will lessen as you continue to use it on battery.
Building a PC is not as daunting as it sounds as it quite straightforward.
1st decide what processor you want the best you can afford really. I-7's the best at the moment but most expensive. Processors will come with there own fan and heatsink assemblies and you will need thermal crease for the contact between the fan assembly and the processor die surface otherwise heat will not dissipate correctly from the processor.
2nd Once you know your processor get a motherboard that has the correct slot/socket for it
Most standard ATX motherboard can take up to at least 16MB RAM usually in 3/4 slots. ASUS or Gigabyte main
ones. Most new ones will have SATA 6GB drive sockets RAID abilities and at least 1 PCIE2 slot for your
graphics card. You can even go ATI cross fire or Nvidia SLI with two cards if you want to go mad, but one is
enough for most but the hard core gamers. Most will have onboard sound processors aswell.
3rd. Ensure you get a case assembly that matches the form factor off the motherbaord you have chosen, ATX most common but it is large (As a guide twice the size of your desktop in work). Consider how many ext drive bays you want for DVD players and internal 3.5 bays for your hard drives.
4th Consider what graphics PCIE2 card you want ensure its direct x 11 as that is most up to date interface with windows. Go for one with 2Gb graphics memory, they should all have HDMI or DVI sockets for connection to monitors. ATI and Nvidia are by far the most common.
5th. The max power you need for your power supply (PSU) will depend on how much power your graphics card needs and very important you get one that has enough power for that. The graphics card takes most power so once you have enough for that you have enough for everything else. Ensure its a good 80+ Efficency model. Corsair usually reliable. To cut down on wire clutter get a moddable one.
Building one is a case of slotting the processor into the motherboard (it will only slot in one way) putting thermal crease on the dire surface attaching the fan assembly inserting the Motherbaord into the case slotting in your RAM and graphics card inserting your hard drives and DVD player into the bays attaching the data cables between them and the Motherboard inserting the PSU attaching the power cables, plugging in the power and reset button and led lights wires from the font of the case to the Motherboard. Tidy the cables as much as you can place the case cover your done ready for windows install (Go for win7 64bit).