It really isnt worth it
I only mentioned the rendering time thing coz u asked if it made a difference (yes it does) but doesnt mean you need to worry about it - especially seeing as its download times that really restrict the size and amount of textures.
(Tho it is worth noting that changing to jpegs doesnt make any difference to rendering time, as all textures are expanded into a raw format once they're loaded into memory, whether they're bmps, jpegs whatever - Using Jpegs jus helps reduce download time and size. (Actually the only exception to that is .dds files which are the directX image format and come pre-converted in the native format - but they suffer from a being a bit large file-size wise.))
But anyway.. if you really want more info...
"i'd guess based on that, a house would be good at 8bit RLE compressed, whereas the radio would be fine as-is. "
Well.. aint simple at all really to specify the rights and wrongs in terms of textures->rendering times really, depends on all sort of stuff.. For instance, comparing the house and radio - Coz the radio is only drawn once, uploading the texture to the vid card is gonna be the most significant part of the rendering time for it, and hence reducing the texture size will make more of a difference to its overall drawtime. The houses/buildings meanwhile - in general - are grouped and rendered in batches according to the texture they use.. the texture is only uploaded once and then its the geometry rendering that'll be the significant time consumer. ( I say 'in general' because there may be cases where this is not always the case - for example, because some ' idiot' (;)) demanded larger buildings that didnt clip I had to strap in some special code that doesnt go thru the normal 'efficient' rendering system... lots of large buildings with large textures would be realllly bad. ) Usually, of course, drawing the couple of polygons used on the radio is gonna take a load less time than any number of houses, but then again, some video cards/PCs are gonna be shinola hot at dumping out geometry but absolutely useless at uploading textures and might actually take longer to draw the radio than a couple of houses (unlikely but possible) - this all can depend on the card type, the screen res, the amount of other stuff running in the background, the supported graphic formats of the video card, the detail level of shadows and any number of other bits n pieces etc etc.
Also, as devi mentioned, the actual rasterization of the texture onto the screen can vary depending on the size of the source texture and the size it is displayed on screen. The PS2 is particularly bad at rendering a large texture to a small space, and im sure the same (or the reverse) is true with different PC vid cards.
The general procedure in most parts of the games industry is for artists to make stupidly high-res stuff which the programmers complain about, then the programmers code somethin in to automatically reduce the res without the artists knowing, giving them somethin to complain about in return when they discover whats happened. This is excepting the extremely rare occasion when an artist produces something lower-res than expected, in which case the programmer keeps very quiet in the fear that the artist will find out and re-do it 4 times larger.
..number of instances, or more of a total-number-in-line-of-sight thing..
In general its whats in view that matters. But then again, on some occasions certain buildings and stuff will go thru the main rendering process even when u can't see em...
Obviously with the nature of this game you'll generally have no idea how many of what is gonna be in view at any one time, so ya jus have to assume the worst.. (Another side-effect of the configurable free-world type thing. Jus imagine how easy a time wipeout or somethin has of it..)
if you've got a good reference site
http://www.flipcode.com is probably the best site for serious developer info.. but as i said, its really not worth it
The main issue worth thinking about here really is the minimum spec thing, and how much that affects the level of the graphics that are made for the game.
This is where consoles make life much easier - if you want 60fps, then you know theres a pretty much fixed limit for the total size of all your textures (Jus a few megs worth on the PS2) and geometry, and hence the textures & models are made and adjusted until they're within the limits. PC is a harder to define coz it starts bringing into play various other issues about who you're aiming the game at and what levels of technology are gonna be required to see it properly.
So in summary, i dont think you should stick to any particular limits except what seems reasonable for ppl to download. Graphically, the game should have the options to override anything particularly silly you try to do anyway..
Oh and dont forget that an 8bit RLE compressed bmp still has a 32-bit palette. It really is suprisingly rare that textures suffer significantly from that compression.
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