i think one of the easiest ways to update the look of your card is to change the color of the envelope.
you can get clear labels from any stationery shop that run through your printer at home and you can use a nice calligraphy font to design them and stick on to your envelopes.
if you have time, are crafty, and are willing to work on possibly a few hundred envelopes (depending on how many you’re sending out), then envelope liners are also a nice touch- How to Line Envelopes | eHow.com
but before you do any of that, you have to figure out if working on the invitations is important to you and your wedding and worth the effort.
for back of card, yes you could make monogram stickers and use them as a “seal” or you can actually have a seal made. http://www.etsy.com/search_results.php?search_type=handmade&search_query=custom+seal&order=date_desc&ship_to=
in the US and Canada, you can also personalize/custom design stamps with photographs or imagery that you can use on your invitations too.
http://www.canadapost.ca/cpo/mc/personal/productsservices/atoz/picturepostage.jsf
http://www.usps.com/postagesolutions/customizedpostage.htm
one more idea that i particularly love is a personalized return address stamp, especially if you are sending out rsvp cards. you can pre-stamp your address and put a custom postage stamp on rsvp envelopes to give your invites that something extra-
custom address stamps on Etsy, a global handmade and vintage marketplace.
and of course, if you do end up investing in custom designed, then your designer should be able to incorporate a cohesive look into all of the above for a jaw-dropping, super gorgeous set of wedding stationery! 