I think the term you're looking for is 'Top Down Shooter'. There's no such thing as a 'Top Down FPS' because FPS stands for First Person Shooter and top down games are 3rd person.
I'm assuming you don't mean 'bullet hell' type games like Ikaruga or Raiden, but more Smash TV, Alien Breed, or Alien Swarm type games.
Your PC should run any game of this type based on your specs. They aren't hugely intensive. You're not exactly dealing with Crysis level graphics or anything. You're pretty safe buying any top down game.
Shadowgrounds is decent.
All Zombies Must Die has a sort of Zombies Ate My Neighbors kind of vibe to it.
Dead Horde is another Zombie game but a more serious and gory take on it. Sort of a top down Left 4 Dead kind of game.
Then there's Arena Style shooters like Geometry Wars and Bullet Candy. No story, just blasting increasingly difficult waves of enemies to gain a high score within a limited space.
Steel Storm: Burning Retribution is a nice medium between the two types and has a level editor as well. It can also be played from a close up 3rd person perspective.
Then there's Retro City Rampage, which plays like the original top down GTA games before GTA III. It's very retro and has 8 bit style graphics, but it's tons of fun to play.
If you're into this type of gameplay, you might like a top down loot RPG like Torchlight or Diablo. Both Torchlight 1 + 2 and the Diablo series have randomly generated dungeons so every time you play it's a bit different. I'd recommend starting with Torchlight if you're interested in trying it out. It's inexpensive and easier to get into. Diablo is kind of hardcore and the first two games are a bit dated, 3 is a lot of fun, but I wouldn't recommend starting with it, Torchlight and it's sequel are a much better way to get your feet wet with this type of game.
There are a ton of top down games out there. Not as many as there used to be, but it's far from a dead genre.
