My 2 cents; you need a better mic long before you need a better interface
I also don't hold the NT1A in high regard. I think there are some much better mics available in the sub-$300 price range, including the Miktek MK300 and the MXL CR-89. Now a Michael Joly-modded NT1A is an excellent sounding mic, but the mod is over $300(it will compete fairly against a U87 after the mod).
MXL CR-89 advantage; very good sound and can be found for around $225
Miktek MK300 advantage; sounds great and has 3 patterns(in a good-sounding room, that can give some great options. I record acoustic guitars with an MK300 in figure 8, but I have a nicely treated acoustic tracking booth)
Modded NT1A advantage; sounds very close to a U87 for a comparative bargain
Food for thought...