Which kind of computer will run faster for minecraft?

Currently the computer I have is a Dell optiplex 780 with a Processor of 3.00 ghz (pretty damn high) but with 3 GB of memory. It also has 150 GB of space. When I run minecraft on it however, it seems to lag badly with default settings on. I put minimum settings to make it run faster and it works for single player but still lags badly on multiplayer.

I am currently thinking of buying a mac online and some that I am looking at range from 2.5 - 2.9 ghz with 4 GB of memory. Also, they are 15" Macbook Pros with older Operating systems like Snow leopard & Maverick. I want to make sure that whichever I buy will run minecraft smoothly with default settings on all kinds of servers. Will getting one of those Macs make a difference or is my current desktop good enough?

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