This would be my advice to you... Just wait a bit until OS 4.5 for your Bb becomes available. (It wont be too far away). BB Smart Email is a great application but the native system on 4.5 looks and feels great. (Heaps of differences between the current and the Upgrade and you will save yourself a few $)(And besides, BB Smart wont work on new OS but this may only be a Beta thing
)
Gmail is a web based (Google) email address server and doesn't make any difference on how you view your email on your device. Google Talk is a chat group application that is becoming native on BlackBerry. Good but only if you like chat group type protocol.
Hope this helps (and saves you a few $$$ )
Kindest Regards,
Greg Myers.