> Will an RC-setup do, or do I need an xtal for accurated oscillation?

You'd have to measure the actual RC frequency to see if it's good
enough. Nominal 4MHz can be 2% off and it varies with temperature
and Vcc. A resonator or crystal would be better

> Also, is there any rule of thumbs regarding the branding of xtals?
> I've got these ones in my junkbox:
> 
> http://bos.hack.org/tmp/cache/xtals/small.jpg
> http://bos.hack.org/tmp/cache/xtals/large.jpg

All except the 1.8432MHz are discrete crystals. The 1.8432 is a
canned oscillator, requiring 0V and 5V, with a 1.8432 output as an
external input for the PIC. Canned oscillators tend to use a lot of
power

If you want it simple, the 2.4576 would do fine for 8-bit timer IRQs

2457600 / 256 = 9600,  00-95 RAM counter -> 100ths sec

On the old 4MHz micros I used to use 2.4576 and 3.2768 a lot

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