Your best bang for buck is front end hardware and a patchbay, or inline with a summing mixer. While a mixer is nice and very organizational, you have to spend some major bucks for quality and to get that big-studio sound. And while most boards and whatnot are transparent not to damage your signal, a Mackie won't give that electronic goodness or the umph one gets from tracking on an old neve or the mix buss on an SSL. Just a professional preamp with good components that will last a long long time start about $400 to however large your bank account is. How many of those things can you put into a $500 mixing board? Again, a low-cost board isn't going to ruin your signal but isn't going to wow you with its sound, either.
I've found that good outboard going in really helps me get better recordings. Of course, I'm a better engineer than musician, so I don't have to spend money on expensive guitars and other such nonsense (tho my guitarist does) and I haven't even bought any new soft synths for a while. Again, an interface pre is perfectly fine for most recording duties, I've found but a nice external unit adds just a hint of air around the recording, a transformer thickening and component excitement that separates each sound itself out in a mix.
For my money, a summing mixer and good outboard gets one the best sound, if you don't have enough room (and money) for a new or old Neve.