Last Minute Upgrade - Low Profile GPU or New Case/PSU/More Cost

I'll try to make this short.

My parents needed an upgraded computer. They wanted to get a Dell, mostly because my father would get a disabled veteran's discount. I walked them through the customization process, but wasn't thinking and let them get a slim case. I also said not to get a graphics card because there would be better options elsewhere -- not considering the effect of the slim case.

Additionally, I bought them a 24" HD monitor. The Dell did come equipped with HDMI, and the machine runs great. The big monitor reminded me about the graphics card.

Now, I've already spent around $500 on upgrading the household's technology -- on a Philosophy student's income. From what I understand, the case will only support certain graphics cards because of its very physical makeup. Meaning, more bang (so Mum can play Spore, my dad Flight Sim X, and I can MMO over holiday) isn't just the cost of a new card, but a case and a new power supply, too.

My question/last minute holiday request is this (finally) : is there any possible way that I could spend comparable money (say, from the $100 low profile to the mid-range $150-$175 GPU) without spending just as much on upgrading the rest of the hardware, assuming a new power supply will be needed either way?

(Edit: This is important, too: being able to display anything at all at 1920x1080. The quad-core, RAM and onboard graphics can actually get 30+ fps on Age of Empires 3 at full resolution ...with the graphics set to 16-bit rather than 32.)