No, no, no and NO... do not despair about the stupid math... I really think that you can raise your score with studying the math... much easier to improve than verbal! And don't change your plans just because of this small obstacle... although it may seem unconquerable at the moment.
if it makes you feel better, I am pretty much in the same boat, although I don't consider myself weak in math I am learning it all over again because I didn't learn it in english... and since GMAT involves so many word problems I tend to be very slow at those translating it into regular, straight questions rather than the given convoluted ones. So for me, math is not so much a fear, but a good time management and learning the unknown terms (reciprocals, primes, etc)
another thing... i don't know about Manhattan GMAT
course online, but I can tell you about GMAX... I am currently taking it now and learned some new things from it... although I have a long way to go.
but no matter what course you buy, you will have to put tremendous effort to succeed... but given that you are aiming only at mid 500's I think that such goal is very manageable.
good luck and keep your spirits up!