I wouldn't do it. It might run for a while, then you'll watch components fail and it'll cost you lots more than just doing it right to start with.
I'm lookin to upgrade the PSU in a couple of months anyway.
If you can have a meter, you can test your current power draw. My system is similar to yours but with a 7870 and I'll easily break 200 watts just basic web surfing. Crank that up to full blown settings and you'll be hitting the limit pretty quickly. If you plan on upgrading down the road you can always buy it and run a slight under clock on it if you start having crashing issues. I do it just because I don't need as much juice as I have on most games. Plus it allows me to run the fan quieter.
If you can have a meter, you can test your current power draw.
Just remember to subtract 15-20% from the reading. PSUs are rated by what they deliver to components, not what they suck from the wall.