Upgradeable cheap AMD build (non-gaming)

I have started this budget build and I’d like some advice for some of the remaining parts (especially the motherboard)

-System usage: surfing the web, office work, many applications running at the same time (no gaming!)
-I don’t intend to overclock
-I’ll be using Ubuntu
-I have no experience building computers so anything would help
-I already have:
>CPU – Athlon II X4 640 3.0GHz
>Power supply – Antec EarthWatts 430WD
>Case – Antec Three Hundred (bottom-mounted PSU)
>Plus monitor, power chord, dvd drive, speakers, mouse, and keyboard

My biggest concern is getting a system that will be a good bang for the buck and will last me a while. I plan to upgrade the CPU to an AM3+ in maybe 2 years (after the prices have likely reduced), but I would like to save most if not all the other parts for longer than that. Here are my most likely possibilities:

Motherboard – GIGABYTE GA-970A-D3 AM3+ SATA 6Gb/s USB 3.0 ATX -or- ASUS M5A97 AM3+ AMD 970 SATA 6Gb/s USB 3.0 ATX

I’m really not sure about the motherboard – these (along with the required graphics card) are pretty expensive compared to my other parts. Will I suffer much performance loss with an 880G board and an AM3+ processor when I eventually upgrade? Or does anyone know – should I wait until the bulldozers are out to see tests? If the older 880G architecture is not going to be a problem with AM3+ I would go for something like ASRock 880GMH/U3S3 SATA 6Gb/s USB 3.0 Micro ATX

Graphics – I don’t really need anything here (I would have been fine with onboard), and I don’t really know anything about graphics cards (I just picked out a cheap one on newegg) – PowerColor Go! Green AX5450 1GBK3-SH Radeon HD 5450 (Cedar) 1GB 64-bit DDR3 PCI Express 2.1 x16 HDCP

Memory – 8GB might be a bit overkill, but I’m thinking it’ll last me through an upgrade – 2x Kingston Value 4GB (2 x 2GB) 240-Pin DDR3 SDRAM 1333

Hard drive – Western Digital Caviar Blue 250GB 7200 RPM 16MB Cache SATA 6.0Gb/s

Thanks tons!