You can set it to anything you want, just remember that the higher you set it, the more RAM its going to consume, and is more likely to push something into the page file and degrade overall performance. The game itself doesn't require a lot of resources, but the way memory management works in java causes it to fill faster then it cleans up. Setting it higher doesn't completely get rid of it, but spaces out the pauses a lot further apart. 512m should work well enough, but I wouldn't bother going past 1g since it lengthens the time needed for clean up operations when it does need it.

As for proc affinity, you can't completely trust task manager since its not giving the full picture. But check the process tab to see if its using 50% or 99% of CPU time, and what else is actively running. I haven't thought about it in a long time, and never got too in-depth, but IIRC single threaded processes will get bounced around all the cores due to the way the thread scheduler in the OS looks at things. Its not parallel processing since its only running one piece of the thread at a time, but is effectively taking turns on each core in a round robin fashion. (Thats where I messed in up in my other statement)

You can identify this behavior by looking at the CPU usage in the process tab and performance graph. If the workload looks to be spread evenly across all cores detected cores, and the CPU usage for the process is 100/X where X=number of cores, then its single threaded app. Multi-threaded processes should be able to break that cap, and may even run an asymmetrical CPU load if its using worker threads to do different types of tasks.