Its pointless to buy a new OEM desktop to upgrade it latter.
It is inefficient money wise because you are paying for something twice (like $35 now for generic PSU and $55 latter for gaming one)
More importantly you run into many problems trying to upgrade an OEM desktop into a gaming PC.
OEM pc's are just not designed for this so the case/motherboard my have specialty parts that are incompatible with aftermarket parts, motherboard is not going to have the same "gaming" features, the cooling ability is limited to say the least, the cable management is non-existent, top mounted PSU instead of bottom, the list goes on.
To sum it up doing it this way will have you spend $400 now and $400 latter to have what you could have built for $600...