Anthony K,
While some video editing - Adobe Prieimere 2014, and programs such as After Effects can benfit from multiple cores, in many cases the peak efeiciency is still at 5-6 cores. A Xeon E5 system would be the best as a first version can use up to an 8-core, has an SATAIII controller and USB 3.
This is suprisingly affordable with a bit of shopping vigilance.
Consider the sales value of your HP xw6600 and adding the upgrade alowance. the maximum sales price for a complete HP xw6600 is about $350,, but if your sysrtem was of a median vlaue of about $250 and another $250 was avaialable there are a lot of options including a z420 with a 4-core E5-1620 (3.6 /3.8GHz) > sold for $250. The K2000 is that particular system is an excellent GPU for your use. That system would be as is quite a bit faster than your current system and having an LGA2011 Xeon E5, for only $70-80 that could become an E5-2670 8-core @ 2.6 /3.3GHz. Over time, the remaining budget could add a good 250GB SSD such as an Samsung 850 Evo- $80 and a WD Blue ITB- very good performing $50.
In my view, that approach- even if it meant buying a z420 and waiting to upgrade is a better long-term tactic than upgrading the xw6600.
The highest rated xw6600 of 183 tested::
Rating: 2609
CPU: 8101
2D: 482 (Radeon HD 6870)
3D: 2765
Mem: 826 (16GB)
Disk: 5115 "Volume 0" - a RAID 0 of unknown drives
For comparison, there are 21 z420 /E5-1620 / Quadro K2000 systems on Passmark, the top rated one being:
Rating: 3798
CPU: 9253
2D: 820
3D: 1658
Mem: 1973 (8GB)
Disk: 4491 (Crucial MX100 512GB)
So a very moderate specification, $250 z420 is already doing very well against the best xw6600.
Condidering that the cost of the Radeon HD 6870 and Crucial MX110 is the same added to any system, the cost / perfromance is better when added to the z420 and the future potential in terms of an 8-core CPU makes the z420 much more future-looking and with a much better resale return when buying the nest system.
Cheers,
BambiBoom