A couple of things here. Like others have said, Faster CPU and good graphics card.
@Laguna Hiker, the GTX770 that Neil pointed out if way too much card for your CPU. If you are looking at upgrading your CPU to the latest i7 then by all means, great card. You can even get away with a GTX760.
@Alvin your GTX650Ti is a good card but it can be the bottleneck here. What is the card running at % wise during renders.
In general, the things I have found to be the biggest bottleneck is disk I/O. You don't have to be able to write at 200mb/s but the amount (number) of I/O's performed is high. I have a RAID 5 array as a source and I render to an SSD drive. My RAID 5 has 1GB of cache and the cache controller will pre-stage the frames prior to reading.
Rendering HD is very intensive, all it takes is 1 bottleneck and the whole process slows to a crawl.
A simple way to see if you have a bottleneck it so watch the CPU and GPU percentages during render.
On software render:
If the CPU percentage is low (<90%) there is a bottleneck somewhere. The lower, the bigger the bottleneck.
If the CPU is high (95-100%) then everything is going about as good as it will get. Look into a good GPU. I would not look at anything below a GTX760
On most codecs the CPU should be maxxed out, or in the high 90%. Some codecs do not multi thread as well, but mainstream 264 should be able to keep it at or near 100%
On CPU/GPU renders.
If the CPU is low (<90%) and GPU is low(<50%) there is a serious bottleneck somewhere else. Start looking at disk, RAM etc.
If CPU is high (90-100%) and GPU is low (<50%) then the CPU is the bottleneck (this is a good bottleneck if you are running on the latest i7 not much you can do about that).
If CPU is low (<50%) and GPU is high (90-100%), well then GPU is the bottleneck. (Not bad if it is a relatively new / powerful GPU, the GPU is earning it's keep).
My $0.02