Hmm. i7-720 lists on newegg for $329 compared to i7-920 at $289 and i7-860 at $280. Don't know about the 820. Are you sure you want to call those upgrades?
I think part of the issue is that the i7-720 is a mobile chip while the others are not. Do you really want to consider a more watt hungry desktop chip for your laptop? The i7-920 has a thermal design power of 130w compared to only 45w for your laptop chip. Think of the impact on battery life.
Also, the i7-720, i7-920, and i7-860 all use different sockets - a major change issue right there.
Laptops are a lot more difficult to upgrade then desktops - both physically and practically. Much more of the builds are customized - interchangeable parts are much harder to find - and you usually dependent on the manufacturer for BIOS changes and compatible drivers for special components. Sorry, but I would never buy a laptop with the intent to make a major upgrade.