For example if it says my build has an estimated wattage of 400, would a PSU with 430w be okay or should I get one with more watts?
Get one with moar wattz. The usual guidelines are 100w extra, or some people stick with percentage extras, and a 500w psu would be 25% extra which is in the range of what the percentage crowd generally suggests.
So I'd specify a minimum of 500w for that build showing 400w on pcp.
So for this build do you recommend the 430 or 500 psu? My budget is tight and kinda stretching already so if I can get away with the 430 would help a lot.
PCPartPicker part list / Price breakdown by merchant
Right, what talby said is right, this one shows 323 watts, so if that is all you are putting in the system the 430 watt PSU should work. My build said 329 watts, and I am using a CX430 - and it's fine even under maximum load testing. That's new of course, and it's difficult to say if the durability might be better with a bigger PSU. But given the price the 500w and smaller PSUs can be had for these days, I just bought one as a spare so if the 430 breaks I'll suffer little downtime anyway.
If you're going to have more parts (e.g. optical drive) as shown in your current saved parts lists, things are different. Your i3 build parts list is 333w which would be fine with the 430w PSU, but the i5 build is showing 373 watts so I would build that with the 500w which is what you currently have on both builds anyway.