I think your old PSU is good enough for the HD 3850.
You would see a major improvement in games. Some games that refuse to install on the old card will work well on the new one.
That being said, you're better off IMO waiting until you can afford a new CPU/MB/DDR2/HD 4850. Something like US$ 400 or 500 would be enough, I think.
Thnks for the promt reply,
Unfortunately I cannot buy a new system at the moment,perhaps then I would be better off with an Ati HD3650 ,a recom 450w Psu & 1gb more ram ..