Titanium Gamer AMTI9025 - It's a decent PC with a decent mid-end graphics card. The processor isn't that great though - it's an AMD (slower inter-core communication, more heat, no TurboBoost, no HT). Although games these days require a 3 or 4 core CPU, with little regard to its speed. The most you need is a video card. With this one you could play at settings close to maximum, at a resolution of 1680x1050, most of the new games. High (not highest) detail in all.
Dell Studio XPS 8100 - it's a brand PC. This gives it some extra cost, but DELL is a good brand, so the assembly quality might be better. The CPU is a lot more powerful. It's a 4 core, 8 thread CPU with TurboBoost. It also has Tripple Channel DDR3 memory, which is also a slight boost from those other 2 PCs. The graphics card however, is much weaker (i'd say 20-30% slower in benchmarks seen across the internet) than in the first PC. I like this one better, but it will perform worse in games than the first one. It's a shot for middle quality gaming in newest, most demanding games, with good resources to make an upgrade in a year or two, to a new GPU. This processor will still be enough to play latest games even in 2 years (unlike the first one).
ACER DEFENDER II - the thing that bothers me is that in its specs, it says Intel Core 2 Quad 9750 processor, which, according to Intel's website, does not exist. A quad core AMD Phenom 9750 though does exist. But it's on an old platform, possible using DDR2 memory (can't tell, specs are too poor). The graphics card is one in the top end. It's a high performing graphics card, although quite an old one. It will drain power, and heat up quite well, make a lot of noise, but perform as good as a mid-end graphics card from the newer ones from nVidia (like GTX 460). And it's overpriced. Definitely a no-no.
Having these details, I'd pick one from the first 2. If you want great gaming right now, buy the first one. If you want quality, and a lot of CPU power, and a fast RAM, with medium gaming capabilities, buy the second, more expensive one.