By Ridge Mahoney, Senior Editor

WHAT A SHOW: A riveting Act I of the four-part weekend series that will conclude the MLS season played out Thursday night at RFK Stadium.

Dallas thwarted D.C.'s desperate surge toward the postseason by beating United, 4-3, in an overtime match of excruciating drama and seesawing momentum. D.C. trailed 1-0, led 2-1, trailed 3-2, and tied the match, 3-3, in stoppage time before falling to Eddie Johnson's overtime strike.

Bobby Convey scored his fifth goal of the season to capture top spot on the team in that category and Ali Curtis came on as a sub to tally in stoppage time and stake his claim for a frontline spot next season. The logjam up top with Jaime Moreno, Eliseo Quantanilla, Santino Quaranta, Roy Lassiter and Curtis will have to be broken up.

THE IMPLICATIONS: Aside from condemning United to its third straight season without a postseason, which will surely accelerate a clearout of players, the result altered the playoff hopes of several teams. Dallas' win gives Colorado additional incentive to beat Kansas City Friday night.

The Rapids trail the Burn by a point in the chase for a No. 4 seed and homefield advantage in the first round. A tie won't be enough since Dallas would win the first tiebreaker — head-to-head results (2-1-1) — if the teams finished even with 43 points.

This potentially helps the Revs and Metros, which are currently tied with the Kansas City. If the Wizards lose, New England can clinch if it ties the Metros Saturday. The Metros would already be in, since it holds the tiebreaker edge over both teams. The Revs would lose a tiebreaker with the Wizards and would also be bottom of the totem pole if it is locked in a three-way tie with Kansas City and the Metros.

Chicago could benefit from a Wizards' loss. It trails Kansas City by a point but would win a tiebreaker versus the Wizards should Chicago and Columbus play to a tie Sunday.

The Fire also holds the tiebreaker edge against the Metros and would come out on top if grouped in a three-way tie with the Wizards as well. On the other hand, Kansas City clinches if it ties Colorado tonight.

There are simpler solutions. Every team still on the bubble — Wizards, Metros, Revs and Fire — will clinch with a win.

''That's our focus,'' said Revs defender Jay Heaps. ''And after the way we played the first half of the season that's about all you can ask for.''

Sept. 19 in Washington D.C.

D.C. United 3 Dallas 4 (OT).

Goals: Convey (Quintanilla) 48, Zambrano (Olsen, Quintanilla) 65, Curtis (Etcheverry, Villegas) 92+; Rodriguez pen. 34, Martinez (Deering, Johnson) 74, Rhine (Vaca) 79, Johnson (Vaca, Martinez) 96.

D.C. United – Rimando, Reyes, Pope, Namoff, Olsen, Alegria (Williams, 68), Etcheverry, Convey, Villegas, Zambrano (Moreno, 67), Quintanilla (Curtis, 77).

Dallas – Jordan, Bonseu, Morrow, Behncke, Pareja (Vaca, 69), O'Brien, Deering, Martinez, Rodriguez, Rhine (Morrison, 87), Kreis (Johnson, 67).

Yellow Cards: D.C. United – Namoff 20; Dallas – Rodriguez 38, Morrow 41, Behncke 91.

Referee: Ricardo Salazar.

Att.: 13,889.