http://www.usa.canon.com/consumer/controller?act=ModelTechSpecsAct&fcategoryid=145&modelid=11939
check out this page, it has the specs for your camera( it's 5 megapixels=5mp btw) and about 1/2 down there is a chart that tells you how many pics to expect from what size card...usually it's best to use the largest finest setting to insure good quality and so you can crop pictures or enlarge them if you want...looks like your would hold about 190 ( give or take some, not written in stone) on a 512 card so 2 gb would give you
roughly 750 photos at the best quality. as you can see from the next chart, don't expect to record any feature length movies
one thing i have noticed as a fairly new digital convert is i take way more with digital than i did with film...it's nothing for me to take a hundred pictures at a setting, i took well over 200 for a 2 day getaway a few weeks ago ( and had i not had my camera on med. quality would have used more memory cards

) where i probably would have taken 1/2-2/3 that with film
there are pros and cons to one bigger card...longer download times if you fill it, if you lose it everything is gone or if it breaks, you are out all also..i have a mix of 512, 1 and 2 gb... i usually use the 512 around here where i am taking a moderate amount of pics and download after every use. probably will use the larger ones for vacation just so i don't have to change it that much and won't be downloading...i'm takin 8 gb for 14 days of vacation but even
i think i am a little excessive in that amount
and it is
killing me not to buy those ones for $9 anewman mentioned but i just bought 2 more g( thought i got a good deal at $30 and here i could have gotten 3 gb for the price..it pains my bargain huntin heart

)