Any newer oSX is going to slow you down, but may be useful, but you need 10.6 & update it to 10.6.78 to go any higher.
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 & Apple Stores, so you have to call Apple to buy it, last I heard.
Call Apple Sales...in the US: 1-800-MY-APPLE. Or Support... 1-800-275-2273
You have to get 10.6.8 first to go even higher to 10.7 or 10.8
You may be limited by hardware in terms of how far you can go.
Kappy 08/2012 post on upgrading to Snow Leopard, then Lion or Mountain Lion - https://discussions.apple.com/message/19401628 - including how to get Snow Leopard and Lion since Apple removed them from the online store.