Ouch, doesn't sound great. Never a good idea to skimp on the PSU. Ask any member here and they'll tell you the PSU is the most important part of the system. There really is a massive difference in quality from those cheapo PSUs that come with cases from competitor sites and a decent PSU from the likes of Enermax, Antec, Tagan, Seasonic etc. I've had cheap PSUs in the past and they've been nosiy, unreliable and have broken easily - taking other components with them. They also have massively exaggerated power ratings. The 400W one you just bought will be probably less powerful than a 300W model from a quality manufacturer.
Sorry to sound so awful, but when you get the funds, I really would invest in a new PSU.