Buying GPU now could be very risky for very little potential profit.
FWIW, I bought 7950s a week ago for a large mining rig to mine litecoin. While bitcoin may quickly move to ASIC mining, litecoin is unlikely to in the near future and has been profitable for me to mine for the past 10 months.
I'm hoarding all the LTC I can right now because I believe as soon as ASICs come to dominate bitcoin the value of LTC will increase by an order of magnitude or more.
I would think you will see the opposite. Once ASICs arrive and the bitcoin difficulty increases by an order of magnitude, if people do switch over to mining litecoins, then you will have a sudden increase of selling pressure for litecoins, driving the price down.
If this occurs, it would be wise to sell your litecoins before ASICs arrive and not after