Your 512 broadband is 512 kilobits per second, which is about 64 KBytes/sec. An 802.11g router has a rated speed of 54 MBits/sec, ie about 7 MByte/sec. In practice, after overheads you get about 1 - 2 MByte/sec, which is still 16x faster than broadband.
Doubling the speed of the router makes the wireless connecion 32x the speed of broadband. However you should note that the double speed is rarely realised in practice - about 20% faster is more likely. Unless you regularly transfer very large files wirelessly, it is not worth the money.
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