Well for one thing, there is no point going for a lower spec video card is there if you can just change the psu?
Loads of PSU's are around that you'll just have to port over. Most cases nowadays are made so the PSU can be replaced without taking the motherbord out first for example, so check how it is stored in your case first of all.
If its fine, all you'll have to do is switch power cables around, and you'll be able to get a much better video card
Even if you do have to remove everything, if you are careful, you shouldn't have to move it that much. For instance, it might slot out if you pull the motheboard slightly outwards, which will just mean unscrewing the screws and being careful not to knock off anything else. If you do this on the floor with the case side down, it might make it easier for you so to not brek off the heatsink connecting to the CPU.
I hope this helps.
*edit* sorry, i must have been writing this at the same time, and yes, those wattages are what your looking for round about. I'd go for a 400watt just to be safe, and might last for another computer update if you were to do one