Would this work or fry everything?

The Dell XPS 710/720 PSU is suitable only for the Dell BTX motherboards is is not suitable for an ATX/non Dell motherboard. Only the Dell BTX motherboard will fit the Dell XPS 710/720 case.

        A Standard ATX motherboard like an x48 board has two power connectors a 24 pin ATX power connector and an 8 pin DC connector.

         The 750W Dell has a 24 pin and a 20 pin motherboard connector, 2x6pin PCI-e, a bunch of sata hard drive connectors, 2x standard molex, a floppy connector and a 6 pin DC peripheral connnector.

         Maybe if you are a skilled rewirer it could work but in the form it is in the Dell layout it won't, the 24 pin connector is identical to an ATX so some modification would have to be made out of the 20 pin connector, it's possible if you know what you are doing but its probably better to buy a thermaltake/antec/ocz 850w for $200+ and get the real thing!

          I have never tired to fit one in another machine but it probably would the screw layout looks the same but I haven't measured with a set of vernier calipers.

          You sound like me trying to work out how much can be salvaged out of the old Dell for the X58 i7 machine in the new lian-li case. So far I've only come up with the 2 opticals and the Western Digital hard drive and the sound card. Though I haven't thought of rewiring the PSU plugs but I could do that it isn't a bad idea.

I am not sure what you mean by 8pin connecter since normal boards if I am correct only require the standard 20-24Pin Main Connector and the XPS powersupply has a 24pin connecter which would power the board.

http://www.newegg.com/Product/Product.aspx?Item=N82E16813121348

              Goto this site and download motherboard manual (before buying a motherboard ALWAYS DOWNLOAD MANUAL I learned this by making the mistake of buying my XPS720 its a good machine but  cannot be upgraded in any serious fashion as BTX boards are now out of production) http://us.dfi.com.tw/Support/Download/manual_download_us.jsp?PRODUCT_ID=5936&CATEGORY_TYPE=LP%20UT&SITE=US  make sure you download English version as there are a couple of chinese language versions and have a look at the motherboard diagram you will see the bopard has 2 plugs the 24 pin ATX and the 8 pin DC power plug (some types of ATX micros sometimes have 4 pin plugs but the ATX standard is the 8 pin power plug).

            I would buy  a new 850 W PSU get a good brad and you will also get the new 2x 8pin and 2x 6pin PCI-e plugs. 

          No the Dell 24 pin plug is identical to the ATX it will not hurt installing it on another board. But the other board will NOT RUN WITHOUT THE 8 PIN ATX DC POWER PLUG.

          This is not an "optional plug" it is essential. The Dell XPS 710/720 is not equipped with this plug!

http://www.newegg.com/Product/Product.aspx?Item=N82E16813121348

Now I see what you mean. Well if I got one of these, it should work?

http://www.short-circuit.com/images/products/cables/4pin-p4-cable.jpg

           I would download manual here for the new intel board and read it thoroughly http://download.intel.com/support/motherboards/desktop/dp45sg/sb/e3001601_en.pdf and goto page 51 "connecting to power" and check the dell XPS 720 has the required connectors I have had a look and am not convinced as Intel are not posting enough information as to pin layout and current, it would require reading the entire manual very carefully prior to purchase but the board is a older p45 board so it is possible Dell connectors or some sort of adapter may work but you would have to make sure the current matches the pins and it is not something to guess.

           The 24 pin connector I know for sure matches ATX as I did this research when I was looking into buying a Thermaltake vga psu to run a GTX280 card I had to make sure the Dell PSU matched the ATX connector on the Thermaltake.

            First rule of computer purchase ALWAYS DOWNLOAD AND READ MANUALS PRIOR TO PURCHASE (IF NO MANUAL AVAILABLE DO NOT BUY) AND CLEAR UP ALL CONNECTIVITY,INSTALLATION, POWER REQUIREMENTS IN ADVANCE. ALWAYS ASSUME SOMETHING IS GOING TO GO WRONG SO HAVE PLAN B READY.

            Remember this is an ATX board and will not fit the XPS720 case.