you have installed the gtx 460m 1.5gb, why you would wish to upgrade this on a laptop is a tad beyond me, the dell site doesn't offer options for it either so you can't opt out or upgrade from purchasing.
given the options you can go from the i3 to an i7, the question is which did you go for?
now dell prices shoot right up for either of the i7's £500 and £1000 respectivly, i gotta be honest at those prices i'd much sooner build a new desktop and use that laptop for traveling.
the thing is when you buy a notebook you should have been made aware of the upgrade restrictions before you paid for it, they aren't designed with upgrades in mind while some claim they can the parts aren't readily available.
dell don't offer the parts on there site either but there may be other methods of getting them, however given the cost involved i'd only go through reputable sources.
the link you provided above isn't exactly encouraging, while i know many laptops have removable cpu's and some older acers have full blown 775 sockets as if they came straight off a desktop motherboard i don't know if other cpu's would even work in the slot.
what you have to bare in mind is the motherboard may not support different cpu's, for example i3 to i7 probably won't work neither would i3 to i5, so you may be stuck with the top end of your cpu range depending on what it is.
