Your miner must not be configured correctly if you're only getting 50-60 MHash/s with a Radeon 7750. That card should get ~120 MH/s according to this:
https://en.bitcoin.it/wiki/Mining_hardware_comparisonAs for "is that a decent amount", the value of BTC in terms of USD most commonly referenced is the last trading price at MtGox:
https://mtgox.comMultiple your BTC by the last price and that's about how many US dollars worth of BTC you've mined.
I'm going to estimate it's not worth it to mine with one 7750, but that depends on your electricity cost; if you have cheap or free electricity it will be profitable to mine on anything. (assuming the hardware was already purchased for some other purpose like playing games)
Find out how many watts your computer uses while mining (it's probably around 100-200, depending on the other components in your machine), then divide that number 1000 and multiply it by 24. Then multiply that number by how much your electric company charges per killowatt-hour; this is how much it costs to run your computer for 1 day. It's worth it to mine if the amount of BTC you can make in 1 day exceeds the amount it costs to run your computer.
Currently the most efficient way to mine is by using an FPGA-based mining device like the BFL Single, Icarus/Lancelot board, x6500, or the Modminer Quad. These devices are relatively expensive, especially considering the fact that ASIC-based mining devices that are an order of magnitude faster than current devices have been announced. No one has demonstrated a working ASIC yet; however buying any type of mining hardware right now is a gamble due to the uncertainty of when ASICs will be released.
Good luck!