What resolution are you playing at? The 5770 will allow you to play most games at lower resolution. I would actually get the 5770 over the GTX450.
At higher resolutions you would ideally want a GTX460 (1600x1020, 1920 x 1080). If needs be, I would sooner drop the CPU tothe RANA 445 in order to get the better GPU as you will benefit more when gaming.