The Z77M-D3H is newer than P61A-D3, because the z77 is the 7 series with Z77 chipset and the p61 is the 6 series MB with H61 chipset. But the Z77 has the older version BIOS when it came out. Yes, sometime you need update the BIOS so that you can use the new GPU. Here is how to check the BIOS version.
http://www.youtube.com/watch?v=xkPUTCjvf_0
I check the gigabyte site 1) the z77 has two version 1.0 and 1.1, the 1.0 rev has 1st BIOS F5 (2012/03/29) and 1.1 rev one has 1st BIOS F9 (2012/07/23), and 2) the GA-P61A-D3 has two version too ( rev2.0 and 3.0) , but the rev3.0 has the FA bios ( 2014/02/14 ), I think you maybe get this version. I know the Gt 730 just came out this year, so the GT 730 works on the GA-P61A-D3 maybe the BIOS.
GA-P61A-D3
http://www.gigabyte.com/products/product-page.aspx?pid=...
GA-Z77M-D3H
http://www.gigabyte.com/products/product-page.aspx?pid=...
And the gtx760 was in the market for while I think it may work on the z77m-d3h, if it does not work and you need update the BIOS, here is how to update the BIOS,
http://www.youtube.com/watch?v=1oyZDhv94k0 I recommend to use the Q-flash method.
But if you buy the gtx760 and make sure you have the PSU can power the gtx760, that requires 500W with 2X 6pin pcie connectors and 30A in the +12V rail.
http://www.realhardtechx.com/index_archivos/Page362.htm Also make sure get the good PSU too. Check out the " PSU tier list"
http://www.tomshardware.com/answers/id-1804779/power-su...
Yes, updating the BIOS has the risk, but if you do it right then it works, so if you are afraid to that, then just get the GA-p61A-D3 ( maybe the 3.0 rev, I call that is the downgrading the MB, but you don't need update the BIOS.) then it will work with the gtx760.