Yes, a Cisco 2821 can do BGP. But as Ron mentioned, it will have a limited amount of memory available to hold prefixes that BGP learns. If you're trying to use two ISPs for internet multi homing, then there are 3 ways to do it:
- Ask your ISPs for a default route: Each router receives a quad-zero route via BGP. Traffic from the internet comes in to both routers, but traffic to the internet uses one provider only. The other router is a backup. This way requires no memory upgrade in a 2821 router, but it's letting a lot of bandwidth go idle. You will have asymetric flows with this method, but it's nothing to worry about.
- Ask your ISPs for partial routes plus default: Each router receives the routes that each ISP directly manages, plus a quad-zero route. Traffic from the internet comes in to both routers, and traffic to the internet will use some bandwidth on the second link, but the majority on the first link, depending on how well-connected your two providers are. This way requires no memory upgrade, but you'll need to be careful that you don't cause your routers to reboot if they run out of memory.
- Ask you ISPs for full internet routes: Traffic from the internet comes in to both routers, and traffic to the internet uses both routers, but maybe not equally. That's okay, though, because it will be best-path, not half-and-half. Basically, traffic will take the path that is best, not the path that keeps your circuits equal. This way will require a memory upgrade for a 2821 router. If you don't get the memory upgrade, your router will reboot continually, each time it establishes BGP and gets the full internet routing table, and runs out of memory.
Therefore, pick the best redundancy that you can with what you have. Or else, get some newer routers. (Those 2821s are EoL!)
Regarding the 890 router doing dual wan, all modern Cisco routers can handle dual wans, but it depends on what you're trying to do. The 890 will not do WAN load balancing, but it will certainly allow you to connect two WAN circuits (with ethernet handoffs) and use them both based upon dynamic routing, floating statics, or IP SLA. It will not allow you to use one WAN for certain TCP ports and the other WAN for some other TCP ports. That's SDWAN.