It depends on what monitor you have, and what Acorn machine you have. There are two main types of PC VGA monitors out there...
Type 1 almost always require separate syncs as the monitor uses the polarity of the syncs (mainly positive going negative or mainly negative going positive) to determine what the line and refresh rate should be.
Type 2 vary. Some require separate syncs (vertical and horizontal) and others will work with composite syncs (vertical and horizontal EOR together).
As PC monitors typically start at a line rate of 30Khz compared to the TV broadcast modes (mode 12 etc.) that have a line rate of 15Khz, VIDC has to do more work to obtain a 30Khz line rate.
This means that you computer will slow down slightly if you use a 30Khz+ line rate monitor. If you have an ARM 3 fitted such slow downs will probably be negligible.
Now, depending on what type of Archimedes you have depends on what type of monitor you can use.
Has software control over the polarity of the syncs and what frequency VIDC is clocked at.
The A540 with RISC OS 2 can only use modes 26-28 (640x480) and 31 (800x600). However, by changing links and a *configure option, you can get the computer to generate separate syncs with no problem.
The other machines with RISC OS 3 can do even better. If you tell the computer that you have a VGA monitor it will re-map all of the 15Khz line rate modes up to 30KHz line rate. This means that you can play your games that require mode 12 / 13. However, as a PC monitor is designed to display 320 lines minimum then you will get a 'letterbox' effect as mode 12 has only 256 lines.
All these machines have 24Mhz, 25.175Mhz and 36Mhz crystals to drive VIDC with. The 25.175Mhz crystal is needed to obtain the correct video rates for PC monitors displaying 640x480 screens. The 36Mhz crystal is used to obtain higher resolution modes, like 800x600x16 colours.
This can generate separate syncs but requires links to be set to determine the polarity. It has only a 24Mhz crystal and can therefore only drive 'forgiving' monitors correctly that don't mind the 640x480 video mode timings being slightly incorrect.
The links to change, to set the sync polarity, are as follows:
Link 24: Change from SOUTH to NORTH
Link 25: Change from OPEN to CLOSED
In order to obtain proper timings, and software control of the sync polarity, you will need a VGA VIDC Enhancer for the A3000.
The situation gets more complex. Due to an 'error' in the PCB / circuit diagram, the A400 series cannot generate separate syncs satisfactorily.
The video signal loses the green component when separate syncs is selected. It has been reported to me that 400/I series machines can have this fault corrected by cutting pin 3 of IC9.
As standard, there is no polarity control over the syncs. In common with the A3000, there is only a 24Mhz crystal. Also it is links 1 and 2 that need changing.
If the PC monitor can handle composite syncs then the monitor can be used in 640x480 mode only.
Most, if not all, of the A300 series had the circuit board hardwired into composite mode continuously. There is only a 24Mhz crystal, and only composite sync monitors can be used. Also the A300 suffers the same problem as the A400, it looses green component in separate sync mode.
However I am told that it is possible if you are prepared to alter the hardware, by fitting a three pin header to both LK10 and LK11 (sited near the RGB connector). Cut the track which connects the middle pin to one of the outer pins of each header. Place a link between the middle pin and the other pin for each header. Finally configure sync to 0 and monitor to 3 to inform RISC OS of the change and you should have separate syncs.
Even after these changes it may be required to cut pin 2 of IC4 to remove the composite sync off the green signal. Some SVGA monitors in particular are fussy about this.
However to connect an older machine (A3000,A300,400 series) to a VGA monitor, you will need a means of connecting the standard 15-pin VGA plug to the 9-pin socket in the computer. A number of companies sell 'converters', but all this boils down to is a lead with the following configuration :-
In conclusion, apart from the A5000 and newer machines, no computer as standard can drive either a fixed frequency or multi-frequency PC monitor in all of the Archimedes modes satisfactorily. However VIDC enhancer boards can be bought to upgrade an Archimedes series machine to handle the needed timing and signals.