Well first realize more RAM doesn't give you better performance as much as just...eh...more working space. Raw performance won't be increased, but load times and all might be as a swap file isn't as needed.
As for having two of the same, you will not loose performance as long as the RAM's the same speed and all as the previous.
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Urban De-Development
* Athlon X2 4200 | 2048M RAM | 160G HD | 7600GT
* C2D T5250 | 2048M RAM | 160G HD | Intel x3100
* G4 1.33GHz | 512M RAM | 40G HD | Radeon 9200
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