The purpose of study is to compare the effectiveness of aprepitant and gabapentin in treating "delayed" (days later) nausea and/or vomiting for patients receiving chemotherapy treatment for cancer. Patients on the study will receive aprepitant or gabapentin in addition to the standard medications used to prevent nausea and vomiting. Aprepitant is approved by the FDA for nausea and vomiting. Gabapentin is approved by the FDA for the treatment of seizures but it may be effective in controlling nausea and vomiting due to chemotherapy. Subjects receiving aprepitant will be receiving a proven treatment for delayed nausea and vomiting, while subjects receiving gabapentin will be receiving a drug that has only shown limited effectiveness.