EC2 uses Xen virtualization. Each virtual machine, called an "instance", functions as a virtual private server. Amazon.com sizes instances based on "Elastic Compute Units". The system offers the following instance types:

Micro Instance
633 MB memory
(up to) 2 EC2 Compute Units (1 virtual core with 2 EC2 Compute Units, for short periodic bursts)
32 or 64-bit platform

Small Instance – default
1.7 GB memory
1 EC2 Compute Unit (1 virtual core with 1 EC2 Compute Unit)
160 GB instance storage (150 GB plus 10 GB root partition)
32-bit platform
I/O Performance: Moderate
API name: m1.small

Large Instance
7.5 GB memory
4 EC2 Compute Units (2 virtual cores with 2 EC2 Compute Units each)
850 GB instance storage (2×420 GB plus 10 GB root partition)
64-bit platform
I/O Performance: High
API name: m1.large

Extra Large Instance
15 GB memory
8 EC2 Compute Units (4 virtual cores with 2 EC2 Compute Units each)
1,690 GB instance storage (4×420 GB plus 10 GB root partition)
64-bit platform
I/O Performance: High
API name: m1.xlarge

[edit]High-Memory Instances
Instances of this family offer large memory sizes for high throughput applications, including database and memory caching applications.
High-Memory Extra Large Instance
17.1 GB of memory
6.5 EC2 Compute Units (2 virtual cores with 3.25 EC2 Compute Units each)
420 GB of instance storage
64-bit platform
I/O Performance: Moderate
API name: m2.xlarge

High-Memory Double Extra Large Instance
34.2 GB of memory
13 EC2 Compute Units (4 virtual cores with 3.25 EC2 Compute Units each)
850 GB of instance storage
64-bit platform
I/O Performance: High
API name: m2.2xlarge

High-Memory Quadruple Extra Large Instance
68.4 GB of memory
26 EC2 Compute Units (8 virtual cores with 3.25 EC2 Compute Units each)
1690 GB of instance storage
64-bit platform
I/O Performance: High
API name: m2.4xlarge

[edit]High-CPU Instances
Instances of this family have proportionally more CPU resources than memory (RAM) and are well suited for compute-intensive applications.
High-CPU Extra Large Instance
7 GB of memory
20 EC2 Compute Units (8 virtual cores with 2.5 EC2 Compute Units each)
1690 GB of instance storage
64-bit platform
I/O Performance: High
API name: c1.xlarge

[edit]Cluster Compute Instances
Cluster Compute Quadruple Extra Large Instance
18 GB of memory
33.5 EC2 Compute Units (2 x Intel Xeon X5570, quad-core “Nehalem” architecture)
1690 GB of instance storage
64-bit platform
I/O Performance: Very High (10 Gigabit Ethernet)
API name: cc1.4xlarge

那請問就技術層面上來說，2位分別不大是吧？我覺得需求是最難估計…但這正正是最重要的…請問你們有什麼方法可以將估計做得更準確一點？

就我"個人"的觀點，這兩家分別在於：
amazon - 很大、很硬，所以該防的他們都防到了，別想從他們身上多撈到好處，除非我們是超超超專業，或是有內線...
rack - 以低價搶攻市佔率，我們若很懂，有機會跟他們弄到一些官網上沒列出且又好康的，比如費用折抵、服務升級...等，但前提是我們要很懂。
但...我們都不是這個領域的超人，所以，我會建議走ama，他們貴(跟其它家比)、硬(條文一大串，規定嚴格)且大，正因為如此，我們沒想到的他們都想到了，大致上跟著他們，也不會出太大的亂子。

前面有位網友Kcars有提到，ama提供一年免費試用，可以考慮，不過有限制，要study一下，若做為評估使用，應已足夠。

至於最關鍵的：如何"評估需求"？
只能說這是大哉問...
因為它沒有一定的答案與方法，小弟簡略地將它分部份考量：
1. 服務類型 - 詳細且嚴格地定義，這個service它所扮演的角色後，才能評估虛擬化是否適合它，虛擬化畢竟不是"實體機"，所有的resource都是與其它vm共享，所以廠商才會為每個instance分級收費，若它吃得重，當然費用也花得兇。
2. 網路流量 - 每家雲端一定也必定會收費的項目，所以這部份也關係到支出的經費。

兩個不太適合虛擬化的service粗略地列表，所以還是特例是適合將它虛擬化)
database
file server

以上為個人小小經驗，建議可以丟回討論串，論壇上一定有神人潛水，若有幸得著他們的指點，就當場增加一甲子的功力啦~!