Have you always been good at math? if yes, then maybe you can simply participate in the forum and see to it that you get good practice everyday. consider participating in our math challenges
regularly and i am sure you will see an improvement in your performance. Otherwise, work very hard on your concepts. A 49-50 quant is not impossible, but needs a lot of work.
my favorite line: ETS makes a living out of developing incredibly intelligent questions. You can never prepare for such questions. But you can definitely stengthen your concepts. Guys like me end up with 48Q not because we are bad at math; its simply a combination of poor conceptual learning and pressure while taking the exam.
I have not used 800 score /deltacourse...so cant talk about that. At this time, your focus should shift away from the temptation of taking practise tests. focus on your errors, focus on your concepts and take practise tests only when you are close to your test date. dont get me wrong, practise tests serve a good purpose; purpose of giving you a feel of the real thing and preparing you mentally for the 4 hour grind. But other than that, never take practise scores too seriously. Its not a good prep strategy to rely on practise scores because its incredibly difficult to develop the kind of questions that ETS develops and its simply too difficult to replicate the enviroment on test day.
one of the options you may want to consider is Manhattan GMAT
's online archives. I have access, but i want to keep it for my reference. Its some tough tough problems. It improved my concepts a fair bit and i definitely benefited from it. its difficult quantify how much a prep tool increased performance, its just my opinion.
you mentioned that you are going to study errors. thats a good thing to do. As i say regularly, i would rather STUDY few hundred problems carefully and completely than practice without keeping track of progress. I think improving by studying errors is the most effective way of practise.
hope this helps