Can you upgrade PSU in Dell xps 420?

yes it can but what do you wanna add that will take more power?

budget range for psu?

I have to assume you have eyes since you're reading this, though you could have someone reading it for you. Most modern Dell's use standard ATX power supplies, but you'd be able to SEE if it was oddly sized.

I'd go to a big seller like Newegg and buy a good name brand power supply, like Corsair's 650W, and stick it in. That is, unless something looks wrong about the way it's going to fit.

We're not building Pentium III's any more.

Before replacing the power supply, I would go to the Dell website and see the which video cards you could order installed on the XPS/Vostro 420. You may find that the card you want to install (or one with a similar TDP) is supported on the stock PSU.

In general, Dell PSU is rated on the MEAN power rather than the MAX, and as such, they can power bigger cards than you might think. For instance, my old XPS 400 had a 325W PSU, but would have no problems powering the GeForce 7900GTX.