Some motherboards/processors use "dual-channel" RAM, when you have two sticks like that. It provides a minor speed gain, but in most cases it is only a couple % faster. I would go with a single stick of 512MB of RAM, as it makes it easier to upgrade to a full gig (which is becoming important now, if you game or edit video).
As for types of RAM, there are two types being commonly sold today - DDR and DDR2. Rambus & SDRAM are types you might see in older machines.
DDR is the most common right now, and is seen in effective clock speeds from 266mhz 400mhz among normal RAM, and up to 566mhz for some non-standard following RAM, normally used by overclockers. DDR is used with both AMD and Intel chips. Your computer probably has DDR RAM, so I would reccomend going with Corsair Value Select or Crucial PC3200 RAM for an upgrade. But since your machine is a new Intel machine (I assume from the 3ghz) there is a chance it might use DDR2.
DDR2 is a different standard, currently only found in motherboards with the Intel 925x chipset. DDR2 supports clockspeeds from 533mhz to 667mhz currently, but due to the higher latencies inherent to DDR2 RAM design, it currently offers no speed advantage over 400mhz, and sometimes the Pentium 4 with older DDR RAM is faster than an identical Pentium 4 using DDR2. DDR2 will scale to higher clockspeed as time goes on. DDR2 is also more expensive than DDR.
AMD is likely to skip DDR2 entirely for their chips, and wait for the DDR3 standards to be set. This is because the Athlon 64 design is more sensitive to latency in the RAM than the Pentium 4 (and the Pentium 4 is more sensitive to memory clockspeed), so DDR2 would offer no performance advantage for AMD, though a nice gain for Intel. That and the built in memory controller in the Athlon 64 makes it tricky to change memory types, though it does provide better performance, especially in games. (Which is why I recently bought and Athlon 64 2800+ - cheaper than the 2.8ghz P4, and it outperforms a 3 or even 3.2ghz Pentium 4 in most games.)
