Phase 1 of this study will evaluate the maximum tolerated dose (MTD) of XL147 when given in combination with trastuzumab (Herceptin) and in combination with trastuzumab and paclitaxel. After the MTD is established for each combination (Phase 2), subjects will be enrolled to evaluate the preliminary efficacy and safety of these combinations in metastatic HER2 positive breast cancer. Both trastuzumab and paclitaxel are used in the treatment of metastatic breast cancer (MBC), but patients can develop resistance.