To further re-iterate on what everyone is saying you can set up the BIS for Gmail or you can download the
Gmail App for Blackberry
For flash websites, you're best bets are possibly to download Opera Mini or the Bolt Browser as they are FAR better than the standard browser however there is still no guarantee it will work properly... Until the Skyfire Browser comes into play (or if RIM get the finger out...) then no full Flash, Silverlight etc support can be found...
YouTube's mobile site does work providing you use Wi-Fi, when i had a Bold on full 3G coverage the quality was nausiating to say the least and Wi-Fi did make that a bit better... There are a couple of apps on BlackBerry App World which are used for YouTube stuff, can't say what specifically as i don't use and ultimately have no need for them as the mobile site does my needs perfectly.
Finally with regards to Tethering i'd be real careful with that one. I called up O2 customer support as i said to them that i was intending to tether my Blackberry to my Laptop occassionally (note the key word being occassionally) and the operator at the other got real arsey with me... Basically O2 would rather you didn't OR would prefer you to get the Mobile Broadband Dongle things, which are utter gash...
Plus using the Curve 8520 as a tethered modem, in my opinion, is a bit daft as it's a GPRS device (or EDGE if you live somewhere that has EDGE coverage)
Other than the above points, the Curve 8520 is a great phone, i prefer this to my Bold...