I will probably be running the new stuff at a little above 1333 mhz due to the limitations of my current ram and wanting to use them in tandem, with CL of ~7-7-7-18. I am wondering what all y'all think would achieve these settings more stably and or clock higher/tighter. The voltage difference is really throwing me off here more than anything.
AMD Phenom II 720
ATI 4850 HD 1GB
ram listed above
Those kits probably can't run together because they are just so different.
Your best chance of getting more capacity successfully would be replacing your current kit with a single kit that has enough capacity for your needs/desires.
If you want 8GB then you had best get a 2x4GB kit (two modules each 4GB) and if you want more than you should get a 4x4GB kit but 16GB is excessive even though it would cost less than $80 if you get decent prices.
Here is a 2x4GB 1600MHz CL9 (presumably 9-9-9-24 timings but might be slightly different like 9-9-9-24-2n or something, negligible differences for performance) kit: