Here's what I'd buy (sorry for the odd formatting, it's a C'n'P job from their site.
SmartValue! DELL™ PowerEdge™ R410 (SV3R410)
PowerEdge R410 Chassis for Up to 4x 3.5" Cabled HDDs, LED Diagnostics, Intel 5600 Series Support 396012 1 [210-31960] 1
Intel Xeon E5620, 4C, 2.40Ghz, 12M Cache, 5.86GT/s, 80W TDP, Turbo, HT, Memory runs at 1066MHz Max 391420 1 [213-11734] 146
12GB Memory for 1 CPU, DDR3, 1333Mhz (3x4GB Dual Ranked RDIMMs) 259814 1 [370-15332] 3
- Extended Service:
3Yr Basic Warranty - NBD Included - No Upgrade Selected 259931 1 [710-16896] 30
- Raid Connectivity:
C21 Cabled - RAID 1 with PERC S300, 2 SAS/SATA Cabled HDDs 356808 1 [780-12017] 1009
- 2 x Hard Drive - Multiquantity:
600GB, SAS 6Gbps, 3.5-in, 10K RPM Hard Drive (Cabled) 364500 2 [400-19708] 1209
- Power Supply:
Power Supply, Redundant, 500W 257941 1 [450-13386] 1015
2x Rack Power Distribution Unit Power Cord 195044 1 [450-11421] 207
- Server Management Cards:
iDRAC6 Embedded BMC 256923 1 [565-10165] 1314
- Network Cards Multi-quantity:
Intel® Gigabit ET Dual Port Server Adapter, Cu, PCIe-4 295325 1 [540-10689] 1230
- Front Bezel:
1U Rack Bezel 257406 1 [350-10513] 669
- Rack Mounting Rails:
Sliding Ready Rails with Cable Management Arm 257399 1 [770-10942] 88
- Anti Theft Device & Asset Tagging:
Asset Tag - Order Details (system type, barcode, Dell Order #, customer PO, shipping date) 300877 1 [293-10020][888-10014] 67
- Base warranty:
3Yr Basic Warranty - Next Business Day - Minimum Warranty 368840 1 [709-10414][709-10563] 29
My reasoning is this - it's a dual socket machine with a single socket full so that you can easily upgrade in the future. I've chosen the cheapest quad-core WITH hyperthreading CPU available, added 3 x 4GB of memory (you could buy this cheaper elsewhere), mirrored SAS disks (I wouldn't trust any SATA disk to run 24/365 myself), a couple of extra 1Gb NICs, rails with cable arms, redundant PSU and a quite useful asset tag.
On this I'd run the free version of VMWare's ESXi 4.1 - as you only have a single ESX server you wouldn't really benefit from the paid version just yet.
I'm no SBS expert but if it's not too expensive to upgrade then I'd imagine it'd be worthwhile.