Looks like the benefit of time hasn't taught salespeople _anything_ at all. Or, customers need to educate themselves before purchasing products for DIY assembly.
Dual Core
A computing technology; multi-core. Meaning, more than one "core" in a processor, also called uniprocessor (single-processor). AMD was the one who introduced multi-core (starting with dual-core) to the mainstream market, but multi-core, more technically known as SMP (Symmetric Multi Processing; also includes multi-CPU systems), has been in the industry for a very long time (not meant for consumers as they were very expensive to fabricate). Intel at this point of time was still in their dreamland of clock speed race.
Intel immediately answered AMD's competition with the Pentium D, which was multi-core based on their "NetBurst" architecture. It failed to win over the market share, but it did OK as the Ds were found to overclock to very high clocks. But at the same time, they produced much more heat. Intel realised that racing for clock speed is a failure in itself.
In layman terms, think of distributing the _load_ between 2 CPUs. 2 cores is not the same as 2 CPUs, so 2 cores of 1.5GHz each does NOT equate to 3GHz of processing power. 2 cores just means your processor load will be shared, but there will be no speed increase by adding another core.
However, sharing the load means there will be more headroom for more tasks. As such, multi-core technology was an immediate winner as it allowed for heavier multi-tasking.
Core (2) Duo; Core
"Core" is a CPU architecture. Totally different from NetBurst, Intel announced this architecture first with their mobile processors. This was known as "Core Duo" for dual-core or "Core Solo" for single-core processors.
Core 2 Duo soon followed for both desktops and laptops, and it proved to be the most successful Intel microprocessor architecture ever.
E5300 is not Core 2 Duo, it is Pentium Dual Core. Lower ranking than C2D or even Core Duo, but higher than Pentium D.
Pentium Dual Core is a revival of the Pentium brand (they were set to be phased out), making use of dual-core technology, but NOT based on NetBurst but rather a different one based on Core.
E6700 _is_ a C2D proc. Pentium D is no longer in production:
http://en.wikipedia.org/wiki/List_of_Intel_Pentium_D_microprocessors
Whoever said E6700 is a Pentium D needs a punch..or just a punch line.
Now about upgrading:
The first thing to consider is whether the new processors would fit your motherboard cpu "socket". Intel, fortunately unlike AMD, has never gone on a socket-changing spree. Until recently, they were still on LGA-775. As such, do NOT get a processor for an LGA-1366 socket, this basically means staying away from anything including or higher than a Core i7.
So considering you're safe in the socket area, the next thing to note is the "FSB" rating of your motherboard. If it is lower than that of the CPU you're planning to buy, then your CPU will be _downclocked_. Eg:
Mobo FSB of 400 + CPU FSB of 533 = CPU FSB becomes 400
Clock speed = FSB x "multiplier" (usually 8, but lower for CPUs/Mobos with higher FSB)
533 x 4 = 2132MHz
400x4 = 1600MHz
Well, you can always overclock
As for the performance increase, even if it's downclocked, a dual-core will outperform your Pentium 4 (which is the shittiest product Intel gave birth to; a time when AMD was king of the world). So go for it, get just the CPU.
