I'm sorry buddy, your PSU is just too low to run HD4850.
Looking your specs, i would say save your money and build a whole new system.
Don't waste your money for this old PC anymore, you'll better with a new pc.
If your current graphics card is connected through PCI express, there is nothing stopping you from connecting a better graphics card, although you need a more powerful PSU. Getting a PSU at 650W is definitely enough, unless you decide to connect 2 heavy graphic cards by SLI or use an crossfire solution.
Your CPU will be a huge bottle neck if you go for a new graphic card such as those you noted down.
So a short answer: get a new PSU or do as the above poster wrote: get a whole new computer system instead.
400W is the peak. It's actually rated for 350W. I wouldn't try to run either of those cards on it. I'd say that the best you can do is a 5670 1GB but even that may be pushing it. If you want something better you're going to need a new PSU.