No change from a Sempron? Have you tried things like unzipping large files, copying a DVD or running Call of Duty or similar games? There would be a huge difference.
You're definitely bottlenecked by your old Radeon HD 3870 card. Although at 1024x768 that should play things smoothly. Did you perform a fresh install of XP, or a repair installation? Have you run a complete virus/spyware scan recently? It's also possible that your long load times are due to an extremely old/slow hard drive.
Higher-frequency RAM has almost no real-world impact on performance, don't waste money there if you've already got a 1333Mhz kit. And yes in dual channel mode, that report is correct.
GPU upgrade first- at least a Radeon HD 5770/6770 although that's overkill for a 1024x768 monitor. Those cards can play smoothly at 1920x1080.
Sounds like you had an older sempron...changed the mother board, CPU, ram, and kept everything else. bet you didn't change the PSU to something more powerful with the 8 pin 12AUX plug. yes in CPU-Z they show the stock data rate not the double data rate so yes you have to do the calculations yourself so it comes out to 1330 or 1333 RAM .. The 32bit XP MCE is killing you..it was designed for playing video not high end computer work nor will it allow you to use more than 3.25GB of that ram. A 3800 series video card (must be from the old machine) is also killing performance. I have to assume the 80GB hard drive is also from the old machine and is either SATA1 or SATA 2 at best...with low cache the Seagate is also a 3GT/s drive...use a 6GT/s
In all your OS, Video Card, PSU, and to some extent, hard drives are killing performance so you "load time" will be slowed up...If you really want FAST dump the 80GB drive...get a 64GB SSD and just put Windows 7 64 bit on it.