John:
I'm not sure, but your quesion
could be interpreted in the sense that you are fond of your T60, and wish to keep it up-and-running, but could do with it being a bit speedier... is that's how it is?
If so; yes, you could upgrade the CPU... either to a Core 2 Duo, and/or to a faster CPU (Core Duo or Core 2 Duo), but the benefit would be modest, unless you really run very CPU-intensitive tasks. If, however, you wish to make a modest investment aimed at "speeding up" (significantly!) your T60, then there's no doubt in my mind about what to do: Get yourself a SSD, provided of course that you don't already have one (in the T60!) already. See e.g. the thread
T60 upgrade question for a few relevant discussions. SSD's for use in a T60 need not be more than SATA-1 speed (the T60 will be the bottleneck), so you can no doubt pics up e.g. an used SSD, and put that into your T60. Even if getting a new SSD, prices have come dramatically down in the recent years, so unless you need some gigant 256++ GB storage space, a new SSD is another option.
My suggestion, if you wish to spend just a
little money, but wish to achieve a
HIGHLY (and eternal!) satisfactory result: Go get yourself a SSD!
If you nonetheless still consider to upgrade the CPU, then I'd suggest you read this forum about actual user-experience with upgraded CPU's in T60's - search e.g. for
T60 CPU upgrade and you soon will be enlightened!
Johan