Unless Dell and HP have included the newer adapters in a hardware compatibility list you will not be 100% sure until you try it. But as your research has shown, the differences are small enough that the newest adapters will likely work in your systems. If it was me, I would buy a couple of the I350-T2 adapters and try them out just to be sure. In the unlikely event that the adapters do not work, you could return them for a refund. See Intel Adapter Money-back Guarantee (North America Only) at http://www.intel.com/support/network/sb/cs-009721.htm for Intel's return policy on retail network adapters.
One adavantage to getting the I350-T2 is that you will get a controller that has all the latest improvements for things like power management built in. If it was me, I would buy a couple of the I350-T2 adapters and try them out just to be sure. Another advantage to buying the newest adapter is that your chances of being able to reuse the system in newer computers and with newer operating systems will be increased.
If you really want a dual port adapter that is PCIe 2.0, the Intel® Gigabit ET Dual Port Server Adapter is an older adapter you could choose. Even though the adpater is older, it still recieves full support from Intel. See http://ark.intel.com/compare/50397,59062 for a comparison of the two adapters.
You might run across the Intel® Ethernet Server Adapter I340-T2 when researching PCIe 2.0 adapters. It's a good adapter, but there is no retail version available. That means if you buy one, you get whatever warranty and return policy and support that is offered by the seller. The retail adapters are the only adapters that come with the warranty and support directly from Intel..
I hope this information is helpful. I would be curious to hear which adapters you end up using.
Thank you, Mark H, I've marked the question as answered.
I agree that the I350 is very likely to work, but I might go for the ET adapter instead just to be sure. (I don't think I need the extra functionality of I350 anyway.)