crashing denotes system instability, not a cpu bottleneck. (there may also be a bottleneck, but bottlenecks cause things to run slower than they ought, they do not cause crashes).
On a side note; low ram consumption really dosn't matter when you consider the features, speed, stability, and security of having a modern OS... not to mention that RAM is cheap (granted DDR2 is more expensive, it is still relitively cheap to pick up a full 4GB 2x2GB kit). On my 16GB ram rig with Win7 I generally never use more than 3GB, even when gaming, so pick up a 4GB kit and upgrade!
Generally instability that causes crashes is caused by a few things:
A bad OC; back off a little if you are overclocking anything (like the new GPU)
bad cooling; Check your system temps, make sure the CPU is under 75*c, and the GPU is under 85*c (they can run hotter than that... but that is not suggested)
Bad ram sticks; run memtestx86+ for ~6-8 hours to see if there are any errors
Bad power management; this is especially suspect as you just added a more powerful GPU, it is possible that your PSU cannot handle it. Remember, it must not only have the Wattage rating required by the GPU, but must also be able to supply the amperage, and a 4-5 year old PSU is not going to give you the optimal wattage and amperage that it did when it was new (shave off 10-15% when doing your math).
A dying mobo/processor; Highly unlikely, but if everything else fails to fix the problem then this would be your last option.
For added performance; will your mobo do a quad core CPU? There are plenty of q6600s and other good cheap LGA775 CPUs out there on the market as people are upgrading. I would not pay over $100 for one, but it may help stave off a full system update if all else fails. Also up that ram to 4GB of DDR2 800. Faster ram clocks does not help much in the DDR3 world, but in DDR2 it will make a noteable improvement.