Hey, recently my Minecraft has been getting very, very laggy after about 10 minutes or so of playing.

I've set these java arguments in my launcher:
-Xms800m -Xmx800m -Xmn600m

I have 4gb of ram, but I don't want it to use any more than 800mb, or even 1gb. I think this should be enough. The F3 menu tells me it never uses more than 600mb anyway.

The problem is, it does use more than 800mb. My task manager shows the memory usage increase, increase and increase. It's at about 1.6gb that things start to freeze up and it becomes unplayable.

I'm really not great with java, and what arguments I should be setting in my launcher etc. So could anyone please help out? I've been struggling with this for several days.

EDIT: Some additional info,
Processor: AMD Phenom 9950 Quad-Core 2.41GHz
Ram: 4gb
Windows 7 64-bit
Java 7 64-bit