The iBuyPower Gamer had a fecal PSU in it; claimed to be 680W but was more likely good for only 400W. You didn't want it.
+1 to deadlockedworld's observations about power limitations though.
You may find the integrated HD3200 is actually able to play some of your older games. You don't mention your resolution. You will no doubt prefer a discrete GPU, but you may not actually suffer too horribly in the interim. You should be able to use a HD4670 on the included Dell PSU without problems. Dell PSUs, though typically the smallest needed for the job, aren't known to be crappy.
If you want a beefier GPU, e.g. a 5750 or 5770, you will want to replace the PSU. For either of those, you don't need more than 500W, and could actually use something as small as an Antec Earthwatts 430. When looking for a PSU, choose one that has full range active PFC (no little voltage switch) and is at least 80+ certified. Antec, Corsair, PC Power & Cooling, Seasonic, and Enermax are among the better brands.