This can also happen if your video drivers are having trouble managing their memory.
Specifically if the F3 screen does not show your Java heap is running with a high % of used memory then heap is not your problem and minecraft does not need more memory.
First thing to check is that on a 64bit OS you are using a 64bit java.
Second thing would be a video driver update.
Well which versions did you install? You can check what windows you have in system specs, going through the control panel. As to what version of Java you have; rather than trying to find it, it's probably going to be easier to install the correct version once you've found out whether you have 32- or 64-bit windows. It should automatically overwrite and replace anything it needs to.
I downloaded version 7 last night.
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There is no signature here, it is all a myth. You think you are reading something but your head is just playing tricks on you.
The thing about getting 64bit java is that the web site tends to not offer it to you.
Get java this way: Start -> Internet Explorer (64bit) -> java.com
Specifically if the F3 screen does not show your Java heap is running with a high % of used memory then heap is not your problem and minecraft does not need more memory.
First thing to check is that on a 64bit OS you are using a 64bit java.
Second thing would be a video driver update.
I'd bet your OS is 64 bit. Please check.
Get java this way: Start -> Internet Explorer (64bit) -> java.com