DDR3 is the type of RAM your motherboard can use.It has nothing to do with graphics card compatibility.You can run a card with DDR5 (GDDR5) memory on a system with DDR1 RAM and there still won't be an issue.
Also PCI Express 2.0 is just the slot you're plugging the card it.It's compatable with PCI-E 3 cards and you will get absolutely the same performance.You will face no problems with it whatsoever.
So yes... as everybody said you REALLY need a new graphics card.The one you're using right now can barely run 6 year old games, let alone new ones.
Remember.More graphics memory doesn't mean more performance.There are a lot of cards that use 256MB of memory that are faster than yours and there are even some 128MB cards, that are faster than your HD 5450.
Whach this video and you'll see just how good your PC will perform compared to a Xbox 360.
http://www.youtube.com/watch?v=ocVyrtcTpHw
Depending on your budget here's what you should to get.
High End Cards :
These cards are as fast or faster than the one used in the video above.
Option 1 :
If you want to use Nvidia only extras and want a really high-end card, go for this one.
http://www.amazon.com/dp/B007VBFDUS/?tag=pcpapi-20
Option 2 :
This card is a roughly as fast as the Nvidia one above, but it is a lot cheaper.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202008&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&AID=10446076&PID=3938566&SID=
Option 3 :
This card is a bit slower than the ones above but the difference isn't going to be noticable.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127685&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&AID=10446076&PID=3938566&SID=
Option 4 :
This card is as fast as the Nvidia above and costs less.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202003&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&AID=10446076&PID=3938566&SID=
Middle Range cards :
Option 1 : This HD 7870 is quite a bit slower than the HD 7950 above but it's still a very good card and it's a lot cheaper.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202010&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&AID=10446076&PID=3938566&SID=
NOTE : these HD 7870 use a "cost-effective" PCB and are known to have problems.
If you don't wanna risk it go with this one instead
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125418&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&AID=10446076&PID=3938566&SID=
Option 2 : This GTX 660 isn't a lot slower than the HD 7870's above.Maybe about 10%.If you're going to use Nvidia exclusive only features, then this one is an very good choice.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814500270&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&AID=10446076&PID=3938566&SID=
Option 3 : This card is as fast as the GTX 660 above which makes it very good value.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131472&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&AID=10446076&PID=3938566&SID=
Lower End Cards
These are cards i personally wouldn't pair with your CPU.They are still more than 10 times as powerfull as your current card, but they aren't nearly as good as the ones above.
Option 1 :
http://www.amazon.com/dp/B009FTWHEO/?tag=pcpapi-20
This card is going to deliver a much better gaming experiance than consoles and it's pretty cheap.
Low - End :
Option 1
http://www.newegg.com/Product/Product.aspx?Item=N82E16814500283&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&AID=10446076&PID=3938566&SID=
This card is quite a bit slower than any of the ones above but its still around 10 times faster than the one you're using now.Its will allow you to play every game at decent settings smoothly but don't expect to max games out with it.