Generally most PS2 games nowadays are DVDs. DVD-R's are preferred, but I have found that DVD+R's work as well. You will need to use good quality media, such as Verbatim or Taiyo Yuden. Personally i never burn games faster than 4x to prevent errors, but some people have success with faster speeds. Of course you will need the DVD Burner as well. You can pick one up for around $30 on Newegg.
Easiest way to check whether or not you need CD or DVD is simple. Look at the back (data) side of it. If it is blue, its CD. If its silver, more than likely DVD, not sure if there are any silver CD-based PS2 games or not.
yo thx 4 all tha comments but i've got 1 more quetion i need 2 ask ya. okay umm.........okay how much space do an backed up ps2 iso file take up if u wonna leave it on the computer hard drive so u can burn it again next time?
well Taiyo Yuden and Verbatim 8X DVD-R would be my top two
Maxcell are ok and should work.
Games are all different sizes so that question would be anywhere from 1GB to 4.5GB for most games.
I've personally used Taiyo Yuden exclusively for gaming and have had no qualms or complaints ever since I've started using them after not having good experience with Sony, but they're still good though. Results typically vary from person to person, PS2 to PS2 and so forth. DVD media quality really depends where you're at, how lucky you are and what's in stocked at your local nearby store, or, if you could, purchase them online if you decide to get Taiyo Yuden or Verbatim. Just because you buy a particular DVD media brand people have suggested, doesn't mean it is always as it is said to be i.e. Memorex is utter expletive but Memorex RICOHJPNs are wonderful actually. Learning the use and knowing how media IDs work can assure you what you are getting is authentic such and such, and also, buying from reputable and reliable sites for your media like supermediastore.com for my Taiyo Yuden DVD+Rs I use for my v15 slim PS2. Check here on a list of media IDs for DVDs.
As for what DVD brands I would suggest, the only 16x DVD media brand I would suggest using would be Verbatim because of the quality results you get throughout the entire disc whether the game is a measly 1 gigabyte or nearly fills the disc around the 4.3 gigabyte mark. I personally use Taiyo Yuden so I suggest those also. There's also Maxell, Sony, and Ritek. Riteks are nice too considering you might be lucky enough to get them under the TDK and Memorex brand if you know where and how to look for them. Sony are pretty good but the results weren't what I expected. Stick to 8x or below DVDs for Sony to get better results, performances and so forth.