Check
***This thread at Dell***. Some people suggest a few types of Core 2 Duo processors to replace a Celeron CPU 540.
There seem to be some of the T5750 and T5800 at eBay for not too much (some for $40 or $50). The T9300 and T9500 are a lot more expensive (starting around $150 and the auctions aren't over). The less expensive ones may be used or refurbished, so check the auction listings.
Both the T5750 and T5800 are 2GHz and there are two processors instead of one, so either would be faster than what you have. Make sure you have a tech who knows what they're doing though. Laptops are a lot harder to service than desktops. A working laptop with a slow processor is better than a defunct laptop with static damage or misplaced parts.
Inland is right that not many adventures make use of both cores at present. Does your laptop seem sluggish?