This is only about 75% of an answer but it seems too much an answer for just a comment:
- Direct connection is supported by Dell for this array
- Every 10 GbE interface I've seen so far will auto-sense 1 Gbps and can be configured to only operate at 1 Gbps
So the short answer is "Yes", and the lacking 25% is the barest slim chance that somehow the MD3800i won't run its 10 GbE interfaces at 1 Gbps, but I really don't think that will be a problem (I can't find confirmation online, though).
If this is a brand new MD3800i then you should have free implementation support with a Dell tech who will guide you through the connectivity all the way to configuring MPIO. If it is brand new and you are just hearing about implementation support for the first time from me, contact your reseller.
It's not clear how saavy you are about iSCSI storage so please forgive this next part if you already know: If you can't get implementation support somehow, then make sure you research best practices for configuring the iSCSI network interfaces. Just off the top of my head (from a recent config very simular to this I recently did), best practice for the R730 iSCSI interfaces will be to max out the Tx and Rx buffers and of course enable jumbo frames. And do not team any iSCSI NICs. The MPIO drivers from Dell will handle multiple simultaneous connections.