DDR2 System memory does not matter for the graphics card. When Your system memory is DDR2, it does not matter if your graphics card is GDDR5 or DDR, it's a different thing and totally compatible. So what I need to see is what is your budget, what is your rest of your system, what is your monitor and what games do you play. Then I can tell you what should you get.
I have a Dell Precision 380, The specs are Pentium D 3.6G, 2GB DDR2 Memory, Max output for any GPU is 150Watts total PSU output 375Watts. I can upgrade power supply and budget is no problem. Games I like to play are call of duty mw3 or others like it. One last thing I want to know is that what about PCI Express x 16 on my mother board because most of new generation GPUs are compatible with PCI Express 2.0.
450 is good gpu but your cpu may bottleneck it,
you can also try hd6670 ddr5 of ati which will be sufficient for playing mw2, mw3 at 1920x1080 at medium settings filtering off at 60fps., but for cod black ops you need a new cpu to get fps above 15.