The 5750 is not a great card and I would not recommend using them in crossfire. You can pick a 1GB version up brand new for around £58 or $100 US. If you go second hand you could pick one up from ebay for close to half that price. Also you do not have to throw away your card, you could sell it on ebay and make around £30 or $50 less fees.
You could then use that money and the money you have for the extra 5750 to purchase a new 2GB card. The HD 6970 2GB, or you could go 2GB Nvidia 560 which is a similar speed but more expensive. If you don't plan to go over 1920x1200 resolution a 2GB card is overkill to be honest, a 1GB HD6950 or Nvidia 560Ti are priced around £170 or $210 in the US.
Both the HD6950 and 560Ti would be faster than 5750 in Crossfire but no multi GPU hassles.
All of this is moot of course if you don't have a decent PSU. What type of PSU are you running?
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