Go big on your CPU+Motherboard+PSU Combo. Snag a GPU in the $290-400 range and you can make it to the 3-year mark before another upgrade which will likely only be a GPU. 16GB DDR3 kits are fairly priced but probably overkill for gaming. An 8GB kit will do just fine for the next 3-4 years. I am running a Tri-Channel 6GB kit and still have no need to upgrade my RAM.
My current rig was built in May of 2009 with a GTX 260 GPU which I upgraded to the 560Ti in the summer of 2011. I will likely be getting a new GPU this winter and continue with this core system another couple years.