You will need a Network Adapter to connect the HDD (the IDE and Power adapter is on the Network Adapter). You can find HDAdvance (what I bought) at ConsoleSource.com for around 30 dollars. Once you have that you can install a HDD (Maxtors work perfectly) any size you want. HDAdvance will say only compatible with up to 120 GB HDD (we'll take care of that later) but you can get as big as you can afford. Once you get HDA and a HDD, download ExecFTPs (google it) and HDL_Dumb (google it too). Find the IP to your PS2 by booting up a game and connecting to the Internet and checking the router log (or asign it an IP with your router <-- what I did). Configure the CONFIG.DAT file in ExecFTPs with your IP (see ExecFTPs's readme). Use CDgenPs2 (google it) to create a BIN file of the ExecFTPs's files. Use CDMage (google it) to fix the BIN file. And then convert the BIN to an ISO. Hook the PS2 HDD to your CPU and use HDL_Dumb to install ExecFTPs on your PS2 HDD. Put the PS2 HDD back into the PS2 and run ExecFTPs and use a FTP program to connect to the PS2 IP. You can now transfer files to the Memory Card, including the Independence Memory Card Exploit that allows you to boot programs from your MC.
That is a shortened version of how to do it. If you want the full tutorial PM me and I'll give you a link. This is the only way to install the exploit which you can then install PS2os and be able to EMUs and ROMS on an unmodded PS2.