I would stick with your current setup: 512K chip on the motherboard, 512K chip on the "A501" trapdoor, and 1MB chip on the MegAChip = 2MB of chip, the max you can have on an Amiga.
Add yourself some fast ram through some other method, such as the side expansion connector or underneath the CPU. There's many options available, including RAM through a GVP or C= hard drive controller mounted on the side, etc., etc. Have a look on ebay. This will give you the best performance (since fast ram added through the side expansion connector or under the CPU will actually give you a performance boost, as opposed to the "fake" fast RAM added through trapdoor). Good luck!
