M98 is just fine
The 98 is an excellent gun, so unless you're a serious tourny player it should meet any needs you have. Example: I was in a local 5 man tourny a few months back; mostly local players, almost all rookies, only two well organized tourney teams showed up-one of those teams won the tourny(no big surprise) and only lost one game-to a team with 4 players using Tipmann m98s(3 had flatline barrels). That team, made up entirely of recballers who only entered because that weekend there happened to be a tourny rather than open walk on play, took second, beating out the second "regular" tourny team. Now these guys are great players, but they didn't need $400+ guns to do well, just a solid well built gun like the 98. The fifth player on team Choad, as they named themselves, was using an autococker, in case you're wondering.
I'm not trashing Autocockers and other higher end guns, they're excellent guns, but they're not really necessary if all you play is recball and a few small tournaments. I LOVE paintball, but I can think of better things to spend a few hundred bucks on than a marginal increase in performance; upgrades on cheaper guns are nearly as good and much cheaper.
As to upgrades, I'd recommend an expansion chamber and an aftermarket barrel. I have some friends who swear by the Flatline and others who just use swear words when talking about it. I have seen them used with excellent performance, though. Quite a few fine barrels other than flatlines are available also.
There are other upgrades such as response triggers that effectively double your rate of fire, and an e-bolt kit, which turns it into an electronic gun capable of 13 bps.
"Just a 98" is not a bad thing, its a very reliable and accurate gun. And no, I'm not defending 98s because I have one, in fact I don't. I use what was once a Spyder Compact 2000 back before all the upgrades...about the only thing left to add to it is an electronic trigger frame.