Hello & a warm welcome to the forums & Macdom!
For Lightroom 3...
Silly Apple, you can't even buy Lion/10.7 anymore, onlt ML/10.8 after you get SL/10.6!???
You need to get 10.6 & update to 10.6.8 first to be able to buy it from the App Store & download it.
Snow Leopard/10.6.x Requirements...
* Mac computer with an Intel processor
* 1GB of memory (I say 4GB at least, more if you can afford it)
* 5GB of available disk space (I say 30GB at least)
* DVD drive for installation
* Some features require a compatible Internet service provider; fees may apply.
* Some features require Apple’s MobileMe service; fees and terms apply.
Which apps work with Mac OS X 10.6?...
It's been pulled from the online store, so you have to call Apple to buy it, last I heard, but quick, call before it's gone!
At the Apple Icon at top left>About this Mac.
Then click on More Info>Hardware and report this upto but not including the Serial#...
Model Name: iMac
Model Identifier: iMac7,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.4 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 4 MB
Memory: 6 GB
Bus Speed: 800 MHz
Boot ROM Version: IM71.007A.B03
SMC Version (system): 1.21f4