You can keep your H700 on slot4. H800 is supported in Slot 3 and 2, so you can connect H800 on slot 3. You can refer "Expansion Cards and Expansion-Card Risers" section on page 123 of blow link for more details on card slot matrix. [URL] 
Hello, I'm looking to expand my server with a PowerVault, but I have an H700 in my R510.  I assume I need to add an H800 to my server, correct?  This will give me the external ports I need to connect to the MD1200 or 1220.  The question I have is if my H700 is the integrated, will I need to remove it and add an H700 & H800 into riser slots 3 & 2?  I'm referencing the note for item #5 on page 131 of the R510 manual that says the installation sequence must be 3,2,1, & 4.  Does this mean I need to fill 3,2, & 1 before I can place a card in 4?  Or am I simply over complicating this?  Thanks.
R510 H700 with H800