A 450w psu will usually include one 6 pin pci-e power connector, and be able to deliver 30-34a on the +12v rails.
If it does not have suuch a connector, then I suspect that it is a cheaply made psu that should be replaced.
I would not try to use a molex to 6 pin adapter.
Here is a handy list of graphics cards and what power needs they have.
Look for a graphics card that will be satisfied with just one 6 pin connector and 450W:
That might be a card as strong as a 7850 or a GTX660ti:http://www.realhardtechx.com/index_archivos/Page362.htm