1 - 99% of the time, laptop CPU and graphics card are not upgradeable. You can play current games but not so sure about future games.
2 - laptop CPU are low voltage type and do not have the same performance as the the equivalent CPU desktop model.
3 - Laptop will get hot and noisy. My Sony Vaio with i7/AMD 6690M get really noisy even when playing WoW. When playing Skyrim the underside is quite warm.
a lot of newer laptops are coming out with the GTX 670/675/680m graphic cards. I'd suggest getting an Ivy bridge I5/I7 with one of the above mentioned cards, it maybe pushing your budget a bit but should be well worth it.
Parts that are replaceable in a "clevo based" (Sager being one of their rebrand reseller) notebook:
and of course Ram, HDD, and Optical Drive.
Basically, most hardware but NOT THE MOTHERBOARD (Chipset).
There are a lot of resellers selling Clevo rebrands too.
Gentech, AVA direct, digitalstorm, eurocom, cyberpower, xotic, sager... (just these off top of my head) etc.
Some retailers also sells MSI barebones, they are somewhat more noisy than the clevo rebrands but cheaper with less upgradeable paths.
cheap but good gaming , LEnovo Y570 at ebay buy a seller refubished for 600US dlls, i would recommend you should wait for more IVY bridge and KEPLER GAMINg LAPTOPS to roll out, just watch the ASUS G75.... AMAZING!!! but not in your budget