The only receivers that dish has wich are hd are models 5000, 6000, 811, 921, 942, any model with a 5xx designation is not hd.
A seperator basicly clones the sat feed signal close to perfect, it looks like a splitter. 942's are a little bigger than your 510 dvr, you can get them from dish for around $600 as an existing customer, one person on here says he has gotten one for $250 by badgering the company repeatedly.
The best way is to take your current receiver thats hooked up to this dish and check in the menu. Take your remote and hit menu 6, 1, 1, select checkswitch and you will see a summary screen that will show your satellites, device type, and at the top it should say what lnbf type. After your find out hit the cancell button until you get back to a picture.