It overflows at 3GB and change, right? Either way, you can remove some of your RAM temporarily to install LR2. If you don't have a single RAM chip/module less than 3 GB, there are ways to force your computer into using less. I know of a way to do it in Windows but not Linux. You can look at this anyways in case you're dual-booting:
http://answers.microsoft.com/en-us/windows/forum/windows_vista-performance/reduce-ram-from-4-gb-to-2-gb/c2089331-121b-4bc2-bae4-ebfc6263ebe6
Just a thought: if it overflows, shouldn't installing more RAM beat the overflow? As in, if it overflows at three and change and you have four installed (so it thinks you have less than 1GB), install one more so LR2 thinks you have more than one GB. Or would this just not work/cause major errors?