You could upgrade on your system piece by piece or two like I did on mine. I started on a cheap but decent build and upgraded part by parts while taking advantage of sale prices on good parts at Newegg.
When I got a video card I got a better made PSU with it to up the wattage. I went from a $200 system to a $600 (my cost) system in 4 months. That's because of my finances and only can spend so much each month (while eating cheap LOL.)
Actually you're better off building your own than buying a ready made because they're out for profits and use cheap parts like the PSU. The PSU is the most critical part of the system, it can run your system for years or it can destroy it.
Your call mate.