If you're using NT-based system, task manager can display GDI objects per process. You can also use my program on http://www.fengyuan.com//download. For 9x-based systems, search MSDN magazine for article/tools.
GDI objects are normally quite small, except for DDB/region/font.
The amount of RAM is not important here, as kernel mode memory is much smaller than that number. CreateCompatibleBitmap creates bitmaps which are compatible with current display mode. So it's sensitive to current display color depth. If you're doing full screen capture, it also depends on current screen resolution.
If you're doing lots of GDI programming,
read my book !
read my book !!
read my book !!!
Even Microsoft engineers are reading it and Microsoft support engineers are recommending to people calling Microsoft.