DDR3 is significantly faster. However the card you have ordered is not intended for gaming and as such it I doubt you will notice any real difference.
If you want a gaming card see if you can cancel your order and tell us more about your system and budget.
but when playing a game i really shouldnt notice the difference that much since my card really isnt meant for it?
No, I was talking about general computer usage. When gaming the DDR2 card will be significantly slower.
I still say see if you can cancel the order or if it is too late return the card. Even a DDR2 HD4650 will be much, much better for gaming and is also quite cheap; http://www.newegg.com/Product/Product.aspx?Item=N82E168...
The HD4650 has 320 stream processors while your HD4550 only has 80. That is 4 times as much computing power. On top of that it has twice as much memory bandwidth due to the 128 bit memory bus(HD4550 is 64 bit.)
As you can see here; http://www.tomshardware.com/reviews/graphics-card-gefor...
The DDR2 HD4650 is a full 5 tiers higher than the HD4550 which is in the same area as integrated graphics. It doesn't say which version of the HD4550 that is on the chart but I suspect it is the DDR3 version so your card is likely even worse. If gaming is your goal I can't imagine you got it at a price that makes sense over the card I linked above.