Regarding "locked" drives, certain older Dells had a BIOS that could only support 2 ATAPI devices. So, you could have 4 HD's, 3HD's and 1 CD/Zip, or 2HD's and 2CD/Zips but you could not have a CDR, a CD, a Zip drive, and an HD, or 3 DVD-Rs, for example. One of the slave ATAPI devices would disappear. This was a limitation of the BIOS from the MB subcontractor, and was not fixable.
RAM can be hard to match, some models take anything, including mismatched DIMM sizes, etc. Other models are very particular about specific supported DIMMs or matched sizes.
IDE devices (barring the issue above) should always work if you have space.
PCI cards should work as well if you have the slots. Like most motherboards, Dells have 'preferred' I/O, IRQ, and DMA settings for slots, so if your PCI device requires or prefers a specific set of resources you might have to juggle there. This is especially the case in Optiplexes. On Dimensions you can generally move existing PCI devices around and put your card in slot 1, so that it gets inited first, but on Opti's with tons of onboard resources all that onboard stuff inits before the slots do, so you might run out of DMA's or your PCI NIC might end up sharing an IRQ with a mouse, for example, which could lead to performance issues. In that case you'd probably need to disable unused items in the BIOS (extra ports, onboard sound if an addin sound card, onboard NIC, etc.) Rare problem, and possible with any computer where you add a bunch of PCI accessories, but could be a problem.