Yikes, that casing looks very cramped. Your PSU looks like a SFF format. Upgrading that rig is going to be a PITA as SFF PSUs don't usually come in higher wattages. Maybe you can consider getting a new casing, then transfer your parts into the new casing, then insert a new PSU and GPU. You can reuse the processor, motherboard, RAM, HDD and disc drive from your old rig. I can always recommend a low profile GPU, but I don't trust that Bestec PSU to last you beyond 2 years and good SFF PSUs are very hard to find.......( and that is assuming that all your parts follow a standard format. OEM makers sometimes uses non standard parts in their rigs which makes upgrading a pain.)
edit: OK, I think I made a mistake. It seems that there are many varients of your rig. Would you kindly measure the width of your case?