Dells can be a bit of a minefield, but most of the time you can work around them
1) Replacing an Optical Drive
Are you replacing a drive, or adding an addition drive to a spare 5.25" drivebay?
If you are doing a straight forward replacement, then there are no problems. Pull out the old drive, take off the rails & put in the new drive with the rails attached. Power/IDE cables plugged in, and your ready.
If you plan on adding a drive to an unused drivebay, you might need to get rails & a Y Power splitter (1-to-2 molex splitter) & a new IDE cable with 3connectors (mobo/slave/master). Open the case to check if u need anything like this.
SideNote - check the chassis formfactor of the Dell your upgrading. The new Optiplex GX620 USSF (ultra small form factor) dont use regular 5.25" Optical Drives. They use laptop-style slimline drives.
2)Replace a Harddrive
Same as above. Might need rails/powersplitter/ide (or sata) cable. You might have a problem finding a space for a HD on the smaller cases.
3)Graphics Cards
Some Dell boards dont even have a slot for a Gfx card. You'll see a lovely hole where an AGP/PCI-Express slot *should* be. If you have this problem, your boned. You'll have to opt for either PCI or stick with the intergrated Intel junk.
Dell also love to use RiserCards. There are equally annoying, most of the time they only support "low profile" gfx cards. Check your board has the RiserCard installed (it wont unless you bought your Dell with a discrete graphics card), do some calculations, and pray that you can shoehorn a better gfx card into that risercard.
4)RAM/CPU
Never had a problem with either of these. Upgrade them as you would any PC. Just double check that everything is compatible.
5)PSU
Never changed one on a dell. Looking at them gives me the creeps.
I wouldn't buy a Dell with the intent of upgrading. it's a handy PC for office work or for your folks. Cheap a cheerful, a good little workhorse for MS Office. But Dell Laptops are very good (apart from the recent occurance of battery blow-outs).
