If you like Yes, check out other British 70's prog bands like Camel. Their 'Mirage' and there self titled 1st albums are a good place to start. Also Gentle Giant is another classic British prog band. The albums, Octopus, The Power And The Glory, In a Glass House, and Three Friends are good starters too.
And Fonzie T is correct, 'Starcastle' were an American band from St. Louis, Missouri , who sounded so much like Yes it's hilarious! Their first self titled LP released in 1976 was their best in my opinion. Their lead singer came from the early lineup of radio friendly MOR rockers REO Speedwagon believe it or not! His name is Terry Luttrell, and he sounds a lot like John Anderson. He only sang on the 1st REO album back in 1971. He really changed his vocal style by the time he joined Starcastle. Very few people make the connection between the two bands.
There are many more great prog bands that I could list, but you just have to search and discover. It's more fun that way. Especially these days with the internet, and so many great sites and blogs to help guide you. Don't limit yourself to to the mainstream prog bands. There's a whole world of great 70's prog waiting for you to discover! Some of the sites I've pasted below might be very helpful for you.
Besides the more common bands mentioned whenever a question regarding prog rock is concerned, there are many other bands from many parts of the globe that are spectacular as well.
Check out the Dutch bands, Finch, Super Sister, Focus, or the Italian bands, 'Arti & Mestieri', 'Jumbo', 'PFM', 'Osanna', or the Scandinavian bands like, Ruphus, Trettioariga Kriget, Tasavallan Presidentti, Wigwam, or the French bands like 'Attoll', 'Hydravion', 'Gong', ' Pulsar', 'Clearlight', 'Heldon'.
These bands are all essential 70's progressive rock bands. Europe had an amazing prog scene, and every country had some amazing music. Especially some of the former Eastern Block Countries like Romania, Poland, Czechoslovakia, Hungary, etc.
The French Canadian prog scene in the 70's was among some of the worlds best. Groups like 'Contraction', 'Morse Code', 'Pollen', 'Sloche' were as good, and in many cases better than many of their contemporaries.
Sometimes it can be hit and miss with obscure prog bands. Many bands such as Camel, Gentle Giant, VdGG, Caravan, etc, had some brilliant albums at the start of their careers (early 70's), but sometimes by the mid to late 70's their material might be substandard, or not up to par with their previous output. This was the case with many prog bands of that era. Many ran out of ideas, and the creative juices stopped flowing (in some cases).
I could write a book on all that stuff. I am a huge prog rock fan. Mainly first wave prog though (70's). I don't care for many of the newer or 'neo prog' stuff. Seems rather redundant and boring to me.
Feast on these sites for awhile!
http://www.gepr.net/geprfram.htm
http://www.progarchives.com/
http://www.progquebec.com/index.html
http://www.italianprog.com/index.htm
http://moonvox.blogspot.com/2006/09/prog-not-frog-radio-presentsthe-voice.html
Here's a great blog with loads of free tunes by several obscure 70's bands. Some are prog, some are hard rock:
http://crotchbat.blogspot.com/2006_09_01_archive.html
You seriously need to sign up and start listening here. The best internet radio stations around. Many great prog stations too:
http://www.live365.com/cgi-bin/directory.cgi?genre=prog%2fart%20rock
http://www.progradio.net/stations.htm
http://www.progressiverockradio.com/
I'm going to see 'Jethro Tull' on Monday Sept 24th. They kick off the North American leg of their tour where I live. They have always been one of my favorite prog rock bands...can't wait!