When building models, have their polygon counts somewhere between 1500 and 7000 polygons for best performance. My graph here has these results from an experiment I did about a month ago:
Code:
+---------------------------+
| object count |
+-+-----+----+----+----+----+
| | | 64| 128| 256| 512|
+-+-----+----+----+----+----+
|p| 10| 78| 91| 100| 102|
+-+-----+----+----+----+----+
|o| 20| 154| 175| 198| 207|
+-+-----+----+----+----+----+
|l| 50| 380| 438| 476| 501|
+-+-----+----+----+----+----+
|y| 100| 720| 830| 910| 950|
+-+-----+----+----+----+----+
| | 200|1330|1510|1660|1750|
+-+-----+----+----+----+----+
|c| 500|2820|3180|3340|3510|
+-+-----+----+----+----+----+
|o| 1000|4310|4930|5050|5320|
+-+-----+----+----+----+----+
|u| 2000|6050|6690|6530|7030|
+-+-----+----+----+----+----+
|n| 5000|6610|6600|6980|6960|
+-+-----+----+----+----+----+
|t|10000|3640|3630| xx | xx |
+-+-----+----+----+----+----+
| |20000|1930| xx | xx | xx |
+-+-----+----+----+----+----+
This graph uses a logarithmic scale. The chart below it has the actual values, in kilopolygons per second, of which are ±2% from the actual, adjusting for the wild variations. The models are flat grids and dynamic. For randomized meshes, it takes 50% longer to render (having 2/3 the frame rate). This is how it is with my system*. The test environment is otherwise an empty level with a single small block (of which can't be seen) and the test subjects. There's practically no script used, only that of what the engine runs by and two very simple camera movement things (only to move the camera faster than what the engine allows as the engine is often too slow.
I do have intentions on running a lot more experiments for more conclusive results on the peak, I've been too involved with my 2D game lately, again.
* My system specs, at the time of doing the experiment, are as follows: CPU = Pentium 4 3.0 GHz with 800 MHz Front-side bus; RAM = 512 MB with 2.5-3-3-6 timing (the center 3's are uncertain), one at 333 MHz and a second at 400 MHz; OS = Windows XP Pro; Video card = Radeon 9600 XT (drivers from August 12, 2004);
Edit: added my chart showing the numbers plotted on the graph.