Does this look like your motherboard: http://www.ascendtech.us/ecs-946gzt-am-lga775-ddr2-sata2-pcie-x16_i_mb4ecs946gztam1.aspx
It's very likely that isn't your motherboard, unless acer's model number matches the motherboard model number. If it is though, it does support up to 4GB. In reality, it's likely that any motherboard from that era (core2duo) will support 4GB (2x2GB).
If you can find the model number (these are often printed somewhere on the motherboard), it would be easier to confirm that it will work with 4GB, but I would be suprised if it did not.
Be sure you get the proper ram type (DDR2 should be correct for that era). If you get PC6400 (800Mhz) be aware the memory may not run at that speed, the speed the ram actually uses is tied in with the motherboard, but the RAM is capable up to 800MHz. You maybe able to save a few dollars getting slower ram, but you don't want to go lower than what your motherboard is running.