This is a Question and should be asked as such. That way you get a E-Mail every time someone answers your question as apposed to a once a day notification to a discussion.
Secondly the Intel DG 31 PR is a Chip Set type of M'Board not an actual M'Board and it's a 945 Chip Set so it's fairly old for a 775 Socket type. You may be better of looking at one of the newer Intel Chip Set Types.
Also as this isn't a M'Board what it can have connected to it is impossible to answer as different makers can incorporate different Features one of which is the Number of IDE Chanel's that are supported. So if this M'Board that you select that runs a 945 Chip Set has 2 IDE Chanel's you can run up to 4 IDE Devices but if it only comes with 1 IDE Connector you can only run 2 IDE Devices.
As for the rest of your Questions most are also impossible to answer as well because there is no M'Board to look at to see what is supported.
But with any current M'Board 1 X IDE HDD and 1 X IDE DVD Burner should work OK provided that you have the HDD as the Master and the DVD Burner as the Slave though the computer may be very slow when you are burning Optical Media.
the 2 GIG of RAM may or may not work depending on how it is actually configured 2 X 1 GIG Sticks should be OK but it may be a Marginal Call if 1 X 2 GIG Stick is OK as for the Brand it may or may not be OK this Depends on who makes the M'Board and what Memory is in their Supported RAM List. Also as this supports a Dual Chanel RAM 2 X 1 GIG Sticks would be better.
As for using 98 SE with this hardware again depending on who makes the M'Board there may or may not be 98 Drivers Available.
Here is the Spec Sheet for this Chip Set Family
http://www.intel.com/Assets/PDF/designguide/307504.pdf
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