Whether the RAM is good depends on what motherboard you have. PC2700 is not the fastest memory. PC3200 is the fastest
generally available memory and is suitable for the majority of P4 and AMD / AMD64 boards. Check your board at
www.crucial.com to see what they suggest you use. Chances are you can take PC3200 with a P42.2 processor. There is faster memory available if you want to replace your board and processor and go for the latest kit.
The graphics card of choice these days seems to be ATI's Radeon 9800. There are plenty of manufacturers and prices are falling. ATI and nVidia have both introduced new cards just recently that are better and faster, but the 9800 should run Half-Life2 and Doom3 without breaking sweat.
To answer your original question about what is the ultimate gaming PC, then yes you want a high end processor (aim for 3Ghz or better). AMD are often portrayed as better than Pentium for games - I can't comment, but I've read one magazine suggesting a Pentium with Hyper-Threading because HL2 supports HT. But the AMD 64 is 64 bit, so twice as much something as any other chip.
You need at least 512mb RAM. I'd recommend 1gb. Many newer boards support dual RAM (I forget what it's called exactly), which allows them to access two identical memory chips much faster. I recommend getting one of these boards and buying two identical RAM sticks at the same time. You can buy purpose-built and matched pairs of RAM, but they cost more and I suspect you wouldn't really notice the difference over two regular RAM sticks.
Graphics card needs at least 128mb RAM, 256 if you can. The Radeon 9800 has become popular because Valve has promoted it for HL2.
Those are the essentials. You might also want to consider networking capabilities on the motherboard, although most boards come with some ability in this area. Also, consider getting a new power supply (suggest at least 400w if you're running a 256mb GPU and a high end CPU) and some additional cooling in your case (you can get a bolt-on cooler for the Radeon 9800 for under $20.
Of course in a few months this will change as bigger, better, faster kit comes into the "affordale" price range, but at the time of writing, I think what I suggest above is a very good gaming machine that won't waste your money. The "ultimate" machine will only hold that title for a few months at best before better kit is available, and the very newest components will always cost a fortune.
