I'm using all the guides and they'll definitely set you up for success if you take the time to go through all of them and digest the material. However, if you're weak in quant as you say you are then check out
MGMAT's Foundations of GMAT Math. I received it along with the other guides from the course that I'm taking with
MGMAT. I'm fairly good with quant so I only read the Geometry portion (totally forgot geometry) and I can attest that the book is well-written and very-thorough.
I will admit though that I have not been just using the
MGMAT guides for quant. I been using Jeff Sackmann's Total GMAT Math, which is kinda pricey at $50 for .pdf file. I took the book to Kinkos, printed it, and bound it together. If you look around the forums, you'll see some positive reviews for it. Personally, I think the book has been a great supplement to the
MGMAT guides. It's always nice to have material that overlaps and I think it's great not only because you're able to solidify concepts but also because you're able to see the same thing from different perspectives.