512MB: RAM size, per stick
1Rx8: The number of memory modules per stick ("1" "R"ow of "8")
PC2: Semiconductor jargon meaning DDR2 (PC3 is DDR3), the type of RAM
4200: Speed grade, 533 MHz here, "U" refers to un-buffered memory
444: Timings, 4-4-4 (CL,tRCD,tRP)
12: CAS Latency (ns)
So the ram you have is: 512MB DDR2 @ 533 Mhz, 4-4-4-12
Your motherboard: a Dell WG261, LGA 775 Socket
NO Support for Core 2 Duos or Pentium D (i.e. no dual core options)
It has four 240-pin slots keyed for DDR2, at a maximum speed of 533 MHz, and a maximum size per DIMM (stick) of 1GB. Timings are negotiated by the mainboard.
In short, two sets of these dual channel modules would fit the bill:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820145527