Research it and get the gig. It'll make a big difference. The machine still won't be fast, but that extra 256mb will help a lot.
You'll only need a few things to replace it:
-A #0 phillips driver
-A T15 torx driver
-Some paper towels or (preferably) cloth rags that you can have ruined (stained at least)
-Some 99% isopropyl rubbing alcohol
-Some decent thermal compound; this stuff, to be specific:
http://www.newegg.com/Product/Product.aspx?Item=N82E16835100007
Read these instructions, except edit in my revision on page #3:
http://www.macworld.com/2003/08/features/15fpimacupgrade/index.php
the revision is where the article says:
macworld said:
first scrape off the old thermal paste from both surfaces with the edge of a credit card. Don’t use an abrasive material, since it may score the surfaces and create gaps that prevent the connection from sealing properly. Also avoid liquid cleaners—they could damage the iMac’s internal parts. When the surfaces are clean, smear a thin layer of thermal paste on the mating surface and spread it evenly
replace it with "wipe the thermal paste off of both sides of the joint with a dry towel/cloth, then wet another section of the towel/cloth (or another one) with a bit of alcohol and wipe the mating surfaces on both sides of the machine (the bottom base and the top of the base) clean. You may need to do this several times until the cloth comes off clean, depending on how neat and clean Apple was when they installed it. Apply the AS5 thermal paste, spreading it with a plastic bag over your finger or with a credit card (it will stain your skin-not harmful, but annoying). You don't need a lot, an almost paper thin layer is sufficient. More is not better.