From what I can see the GD55 has better on board audio and 8-pin CPU power where the GD53 has 4pin so I'm guessing the GD55 is aimed more towards the overclocker.
On the site that I checked it also said that the GD55 has SLI/CrossfireX support but it didn't say that on the GD53 though I'm assuming it does. If you want to be sure; check the MSI website
Well, imo... All onboard audio is quite... bad. Any cheap sound card will be 10x better so if you go with the GD53 you might have some money left for one?
Considering you're buying a 3570k (K stands for unlocked) I was assuming you'd be overclocking. If not you could go with a regular 3570 and save 10-15 quid