this isn't a lecture, and it's not meant to be preachy, but it'll prolly come across that way. there are some "universal truths" out there, of which you don't seem to quite aware, so bear with me here and pardon my occasional sarcasm or bad joke.
first, i have to second chessrock's suggestion of the search function. you'll get FAR more usable information out of it than you will by asking questions. trust me. things you've asked and are likely going to want to ask have prolly been asked before (let alone several times). just plug in the piece of gear or whatever that you want to know about and let it rip. you'll have to do a little more work this way, but you'll be rewarded for your work in the amount of info you get back.
why "search" instead of ask? it's easy--if you ask, the only people who'd answer your question are the active people on this board. there are FAR more members that have come and gone over the years, and using the search function lets you draw from their opinions as well as the currently active membership. a much larger sampling pool, in statistical terms, if you will.
secondly, the only way to tell what mic is gonna work for your voice or guitar is for you to sing/play through it. we could recommend mics all day (and prolly all night), arguing about capsule size, thickness, the antics of the company's president, neumann's pricing policies, whatever, til we're blue in the face, and we STILL might not find something that works on your voice or guitar, let alone both.
third, you've not said anything about the rest of your signal chain. do you have phantom power? what kind of preamp are you using? keep in mind that your preamp will greatly affect the way the mic sounds. sm57's sound a LOT better through a really high-end pre (think API, Neve, etc) than they do through a behringer board or nady pre. conversely, a $2000 neumann will sound a lot worse though a lower end pre. that might sound like a "duh", but it's something a lot of folks don't seem to realise when looking for a new mic. for some, changing mics might not necessarily be the "right" direction to go in. thus, you've gotta take your whole signal chain into account when looking at "what mic next?"
ok.....as for suggestions.....i like the thought of the AT4033. good overall mic, usually sounds pretty damn good on guitar and vocals (among many other things). don't have one, but i have used one and it's on my list when i start looking at LDCs again (i'm going through a "nice dynamic" phase right now). they go for around $399 new, but used they're a little more manageable. so it's a little out of your budget, but would likely be well worth the extra month of saving.
another option is the already mentioned SP B1 (or a pair). as richie said, it's basically the SM57 of inexpensive LDCs. it's *ok* on my acoustic (i like my 603's better for most of the guitar things i do), and the B1 sucks on my voice (but it's good on a couple other voices i know), so YMMV on that one. but for $80, it's a solid workhorse (use mine all the time), and that's something you can NOT say for the majority of sub-$100 chinese-made LDCs.
another thought is an MXL603 or an Oktava mc-012 (both SDCs--usually $50-100 each) for the guitar (or a pair of em), and using the sm58 you already have as a vocal mic. then save up for a "better" vocal mic. the difference between my B1 and my pair of 603s on my Martin was stunning, with the 603s sounding far better. some folks (and guitars) like the mc-012s better. YMMV.
some might recommend that you put that $200 into a 6 month CD and saving another $200 and getting a nice used dynamic like an RE20 or SM7 (among others). plus, at $400 you'll have a lot more choices of "traditionally better" mics in the LDC area.
hope this helps some,
wade
