Hey Ryan, an single HDX system is approximately 2.1 times stronger than an HD3|Accel, so you should definitely feel the difference. On ProTools 10 HD, you will also FINALLY be able to use the Maximum Delay Compensation of like 16k samples, instead of the Long at between 4 and 5k samples.
And, you'll have an extra PCI slot!
A BIG thing to remember and look into is plug-in compatibility. Here's a link to a list of current AAX supported plug-ins (not sure how often it gets updated): AAX Plug-ins List (Avid and 3rd Party) - Avid Audio Forums
If you switch to HDX, I believe TDM plug-ins will no longer be supported. Only RTAS and AAX are supported under HDX systems. Someone, please correct me if I'm wrong. Some manufacturers have talked about making the AAX switch - some already have. I believe Waves said that they aren't jumping on board with that at all, so if you use a lot of Waves, you might have to rethink the switch or find alternatives.
Hope this helps. Best of luck!