Render time depends in most cases on CPU speed and number of cores, more than anything else. Graphics cards are used in realtime views (previews) of the scene, but not actual rendering (at least in the case of most rendering engines, including Mental Ray).
In the case of your Dell T5500, you have a single 2.13Ghz CPU with 4 processor cores. Xeon's do not in general outperform consumer-level chips clock-for-clock. The E5506 CPU is Nehalem-based, so it outperforms the previous generation "Core 2" technology clock-for-clock, but you can see that when the Core 2 has a clock speed advantage, as in the Q9450 result at 19m3s, the older Core 2 technology can still beat Nehalem. It's important to note that this is with a 500Mhz+ advantage for the Core 2 CPU however (2.13 vs. 2.66Ghz). You can also see that the slower 2.33Ghz Q8200 falls well below the performance of its faster cousin, despite having more RAM; this shows that RAM is not a significant factor in this test. I would say that the CPU you have is performing about as it should, given the comparative results we have.
If you want to improve performance significantly, you will need to upgrade the CPU. It's not clear from your description of the machine, but I am assuming it is a single CPU model. That being the case, all you can do is upgrade to a faster Xeon, preferably one with hyperthreading which 3DS Max/MR can take advantage of to improve render times 20-30% (as compared to an equivalent clock rate CPU without hyperthreading). You could for example put in an E5640 which would give you an additional 500Mhz+ per core as well as hyperthreading. Or if price is not a concern (perhaps you can return the GTX 480 and put the cost toward the CPU upgrade), then go for a 6 core/12 thread (with hyperthreading) X5650 giving you not only the extra 500Mhz+ for each core, but an additional 2 cores as well. That ought to really give you a render speed boost.
I hope that helps.
- Oshyan
