aye, well, kinda. a pro tools hd rig, or one of the firewire dsp things you can get would do some of the processing but a normal audio interface won't. pc soundcards have midi engines and stuff on board which are fine for games and simple things, but for music making you'll be running your chosen sample library rather than a generic midi sound. they will allow you to reduce latency and some good stuff like that however which is very important and can make a notable difference in your apps' performance.
a good audio interface is definitely a worthy purchase though. as a guitarist they are useful because a standard sound card doesn't have an input that can deal well with the high impedance signal of most electric guitars. you can still get a line in but you sacrifice a lot of quality. the reduced latency mentioned earlier is very important for playing and getting effects processed in real time. playing a note then hearing it play back just a fraction of a second later can really muck things up. and obviously you can get a variety of extra inputs for recording multiple sources at once, better quality mic pre-amps, converters etc.
I've never heard of minimonsta and my midi experience is _very_ limited but that sounds very odd indeed. a phone should have the horsepower to do what you're trying to do so i'd guess that there is some sort of software configuration issue. i've been able to tinkle away on my keyboard into logic pro 7 quite happily on an old dual 2ghz g5 and even in the dim and distant past i think i did much the same with my old 1.25 ghz g4 imac.
finally, as for native instruments... i LOVE them

i auditioned the demos of guitar rig 2 and amplitube 2 last year and fell in love with the GR software. amplitube was cool as well, but the interface, whilst designed to mimic a traditional guitary setup, seemed horribly limiting compared to GR and apart from amplitube having a harmonizer effect i felt that GR had it beaten in every respect. and loop machine rapidly became my primary composition tool for working on the evolution of musical ideas, riffs and melodies and the like. after checking out the awesome ubersonic amp in guitar rig 3 i decided to upgrade to that too. i've had minor issues getting the software fully installed and registered etc. and last time i checked (a while ago, may have been fixed) you couldn't install gr2 on leopard, but once everything is on and working it is like a dream. you can get the demo of GR3 on their website. I'm afraid i can't comment on their other software, but it's a start