Just my 2 Cents, may this help someone in the future.
I am using a XW8400 for gaming, and all but the most performance hungry games run smoothly. My favourites, Far Cry 4 and World of Warships may not be top-notch titles any more, but I mention them to show where the ceiling is, especialy Far Cry brings the processor to the limit.
I have recently swapped the original pair of X5150 for a dirt cheap pair of X5365 from China. The machine gets a notable performace boost, but not so much as I hoped, considereing I put in 8x3,0 GHz cores compared to 4x2,66 I had before. Looking at performance monitor you can see that Far Cry does put its main load on one single core, so it cannot benefit too much from many cores. Like many games it needs the fastest single core performance you can get. Nevertheless there is a notable performance boost after the CPU swap.
The main component I had to change was the GPU, the original NVidia Quadro is not a good gaming platform. I swapped in a Powercolor HD 7850 (2GB) I had laying around and got reasonable gaming performance for a low price. I do suspect that further upgrading the GPU would give me even more performance, if I had any demand for it.
It has also been a great improvement to put Windows on a quite tiny (64Gb) SSD boot drive, this makes the machine become much more "responsive", the original magnetic drive is still in the box as D: and stores the games.
The only real pain with the XW8400 is the exotic FBDIMMs it requires, not only they are still quite expensive, they also run red hot and require a noisy fan for cooling. I luckily was able to buy a 16GB (4x4GB) bank off Ebay on a defective XW8400 motherboard, so I got mine for 40 bucks or so. Note that 8GB FBDIMM modules won't work, but the 8x4GB supported will give you 32GB max, should be enough for the near future
Armin.