Bingo. With 4GB, a part of that 4GB is wrapped above the 32b boundary, which causes the storage drive to blow it's brains out. I bet if you watch your Activity Monitor, memory usage, it will happen very repeatedly when you get to about 3-3.3GB of RAM used.
Solution: Go into your BIOS. Disable the option to map memory to 4GB. Depends on motherboard, but might be termed "DRAM recovery", "DRAM Remap", or such. On my Asus P5KC, it's "Memory Remap Feature". Set this to disabled. Instant stability. When you boot, it should show ~3.3GB of DRAM. Yes, you just lost 700MB of DRAM.