Hello.
First, what's your PSU?
Second, 4 GB of RAM is good now but it will be not in the future. Even today some games are limited by 4GB RAM. Grabbin 8GB RAM is like $15 difference from 4GB.
Third, the two cards cannot be compared because 5970 is a dual GPU and it has 2-3 times better performance than 550ti. But that might be the same on the price and the power consumpion too.
Generally if you can buy the 5970 (I suppose it will be around $350) you should better go for a 6970 or a GTX570. They are single gpus, offer 80% performance of 5970, but they might be cheaper, consume less power, have no problem with drivers, both offer more VRAM (2GB is a lie on 5970 since it uses 2x1GB in order to maximize performance, so its like a high performance 1GB) and you will not have microstuttering.
If you can't then GTX550ti is good for its price, but 7750 is better (as performance / price ratio)
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Now since you will go from Conroe (1st gen core) to Sandybridge, you will need also a motherboard. (I said that cause you (anee1712) said that you dont know much about pcs.)