I am running lightroom 2something.
Model Name: MacBook
Model Identifier: MacBook2,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 4 MB
Memory: 1 GB
Bus Speed: 667 MHz
Boot ROM Version: MB21.00A5.B07
SMC Version (system): 1.13f3
Serial Number (system): 4H7063XPWGP
Hardware UUID: 00000000-0000-1000-8000-0019E3360C71
Sudden Motion Sensor:
How do I upgrade to lightroom 4?
Where do I find the most cost efficient upgrade, if there is one?
Via Adobe you could go direct to Version 5 by purchasing the upgrade (provided your hardware and OS are supported)
You could find a retailer who still has Version 4 in stock and purchase it from them. (again, provided your hardware and OS version are supported)
Looking at your system specs provided, I don't know if you meet all the minimums or not. What OS are your running?