For the most part, lt_dan's pretty much right. If you can afford it, get the 7850.
But I can honestly not find anything on that PSU - is there any way you can get a photo of the label on it (where it lists the amperages and voltages and such) so I can get a better picture of what it's capable of? I kind of get the feeling it's not really a 450W.
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
The Treetop Corp. TA80 450W PSU is only a 390 Watt continuous rated PSU.
The OEM seems to be Acbel Polytech. Acbel Polytech has a couple of 390 Watt PSUs that have a maximum combined +12V current rating of 29 Amps and they only have one (6+2)-pin PCI Express power connector.
I agree with @geofelt's card recommendations.
Just be careful if you're thinking of getting a Radeon HD 7850 because some of them require two 6-pin PCI Express power connectors.
Honestly, i don't half understand what the table and figures means so i beg for some guidance.
But i do have one 6-pin PCI-e power connector inside, so is it enough to power the HD7850?
You should be OK.
Your psu is capable of 30a. (take the combined 12v watts and divide by 12v,)
You should be ok with a normal 7850, assuming that it is not one of those heavily overclocked versions that need two 6 pin connectors.