It's either T7200/7400/7600 or T5500/5600 - take your pick.
Former have 4MB cache, while the latter have 2MB. Prices reflect that pretty well.
I'm about to be in this game as well since the long-awaited and coveted Z61m came today! It was originally highly spec'd with the WSXGA+ screen, camera etc., but is dumbed-down. Right now, it's running a Celeron 420, so I'll be upgrading that like the OP.
As usual; deferring to you in all things "A" to "Z"
, how do you see the cost vs. benefit ratio between having those extra 2Mb of cache and seemingly doubling the price for that priviledge?
Family Daily Drivers- T430s, T530, X220
Work- Sadly, the ThinkPads have gone away...... and replaced by HP ProDesk SFF drone machines
Other Projects- Edge 15, Z61m (Titanium)
Historic Retired ThinkPads- T42p, X20, A31p, 701c, 760XD, WorkPad C505