I don't want to buy new PCs, as my three Sony VGX-XL systems are really beautiful "vintage" computers, still today with very good qualities that cannot be found on normal PC without a quite big budget... so, no "fancy" Intel I7!

But, a Core 2 Quad Q6700 at 2.67GHz (the best CPU that Sony's motherboard and PSU could handle, and with no overclock, I'm sorry to admit) will be a huge improvement in speed.

I'm aware of no further speed boost by upgrading the GPU, but as the other two old videocards are dead, I think a better GPU than a "simple and honest" HD 5450 will give some modest speed improvement - after all, as I should replace them, I'll go for the best it could fit, e.g. GT 640 2GB or HD 7750 (1 or 2 GB).

I was also aware of the CUDA related things, but I was surprised that an AMD Radeon could improve avisynth/virtualdub speed too!

About SSD and/or RAID: I could do both, but after I will upgrade CPU and GPU, I think the speed boost will be relatively small (5%? 10%?) - a RAM booster maybe could be a cheap alternative with comparable results, what do you think?