|Looks like you keep buying cards that are all in the same performance/price range. If you already have a Radeon HD 4650 w/DDR2, there's no point replacing it with another with DDR3. The HD 5550 drops somewhere between the 9500GT & the HD 4650 & although it is more modern in terms of DX, OpenGL & shader support, it shouldn't be considered.|
Have a look at the following table & make sure to pay attention to what's written at the top, especially this: "I don’t recommend upgrading your graphics card unless the replacement card is at least three tiers higher. Otherwise, the upgrade is somewhat parallel and you may not notice a worthwhile difference in performance."
If you can afford it, get a card with GDDR5 memory.