You are right in that I have a cheap MIG (with only 2 heat settings).
Usually I use flux core except for stainless because its just faster to do a
small repair weld. My business can justify the expense of a better MIG
welder, although an autodarkeneing shield is probably going to come first.
I know the duty cycle of the one I have is abysmal. This one mostly gets
used for minor fabrication for my business. I probably can't justify top of
the line. Anything over a couple thousand would be pretty hard to justify.

The good news is that pricing on power tools is falling further every
day. Most of the stuff from china is not yet up to spec, but will be
in a few years. CEA makes some pretty good welders that offer
excellent value for money in my opinion. We have a whole range of
welders at work, right up to 800A Lincolns, but we just got in a couple
of CEA's new three phase units, i think they're rated to about 400 or
maybe a bit more off the top of my head. They only cost AU$1200
including sales tax which is about a third of the price of a comparable
lincoln over here. I know they have some fairly good single phase
units available in aus for AU$800, so i imagine US$500 would buy you a
lot of welder right now. Three phase is great if you have it, if not,
you can still do fine with single phase. Look for as much adjustment
in the voltage as possible, at least a coarse and fine knob. Dont get
too concerned about things like burn back and ramp up etc. Most
welders have adjustments for these on the PCB anyway. Try to get one
with a universal gun fitting rather than a built in gun. You can
change them to the universal one later if you have to but its ugly to
do so. If you're stuck with the built in gun you'll probably find that
its not up to the task and doesnt have the range of consumables you'd
like.

If you can afford to spend a bit more, you can get some wonderful
do-it-all inverter boxes nowadays for under $2000. These are an all in
one pulse mig/stick/arc/gouging unit with more adjustments than you can
poke a stick at. Most are fully digital. Have a play with all the
functions that are important to you (especially TIG if you are going to
use it) before shelling out your hard earned - each unit has its own
quirks that you'll either love or hate.

In terms of helmets, the good stuff is still pricey. I've used most of
the speedglass units including the top of the range big screen models
with all the adjustments, sidewindows and the ventillation system.
Work provides them to us for work where we get exposed to fumes. I
have a lincoln dominator which i love, it was about half the price of
the top of the line speedglass (cost me about AU$450) but i find it
much more comfortable to use. it has most of the same features (9-13
shade adjustability, sensitivity, delay) lincoln claims that their
helmet has a faster reaction time which is more suitable for tig, there
may or may not be truth in this. I do mostly mig and a little tig with
mine, but i find that when i wear the speedglass i tend to suffer more
from eye strain. most welders prefer the headgear on the speedglass,
its lighter and a bit more comfortable. One thing i find nice about
the lincoln is that in its 'off' state its slightly darker (i think one
shade) than the speedglass. While this may sound silly, it makes it
the right shade for oxy welding/cutting. I really hate oxy glasses, so
this is a nice feature for me.

Its definitely worth having a go with welding the aluminum with your
current welder. You'll learn a lot, and there are some great
newsgroups that are welding/metalworking focussed. The guys on these
lists have consecutive lifetimes of experience in the industry and you
really learn a lot by talking to them.

Good luck with it all~
Shaun