Use a Sony CD/DVD as external DVD ?

yes, you can buy drive enclosures to convert internal hard drives or optical drives for external plug-in use. for example try a place like Otherworld Computing, they have a great selection of drives, enclosures and related accessories and have great support for Macs too.

looks like the Sony DVD has an IDE interface and your hard disk is a 2.5" SATA one (WD1200BEVS is the model#), you'd need to get the appropriate enclosures for each kind of interface and with USB2.0 external interface. because of the different form factors of the DVD and HD, I'm not sure if a single enclosure to hold both exists or not.

for the DVD drive, you might need to check if a special software driver is needed - I'm guessing no, but I'm not 100% sure. but certainly if you convert the HD to an external drive you will be able to use with your MB.

the RAM depends entirely on what kind it is. you'd have to check the specs of both the RAM from the Dell and what's required for your MB and see if they're compatible. the part number is usually marked on the memory module.